The simultaneously frustrating and convenient thing about all of the groups your asking about is that (in a very rare turn of events) the linguistic, archaeological, and genetic evidence tends to agree or at least correlate. The slightly inconvenient thing is that more of the linguistic terminology has made its way into popular discourse while also being distinct from the terminology preferred by both archaeology and genetic studies. This is partly due to the fact that without written records we can't actually prove definitively that a given archaeological group spoke a specific language but genetic research always knows which archaeological sites their samples come from. So first a bit of terminology that I think will start answering the question.

Based on all available evidence the Yamnaya Culture in and around modern Ukraine is identified with the Proto-Indo-European language (c.3500BC). The spread of Yamnaya cultural/genetic influences (c.3300-2500 BCE) is usually identified with the spread of the Indo-European language family. The Indo-Iranian language family represents the largest eastward component of that process (2500-2000BC). There are several material cultures east of the original Yamnaya group that represent the spread of Yamnaya influence before the Andronovo culture. The one most closely associated with the Proto-Indo-Iranian language is called Sintashta. Eventually those smaller post-Yamnaya cultures were absorbed in the larger Andronovo style (2000-1500BC).

Around 1700BC, the Indo-Iranian language group began to split, we know this based on both archaeologically and historically documented migrations as well as linguistic evidence from the Sanskrit language of the Rig Veda. The group that split off is commonly identified as Indo-Aryan, Vedic, or Old Indic. Out of habit, I will use Vedic. Part of this group moved southeast out of Central Asia and into the Indus River Valley and beyond, bringing early Sanskrit and Vedic/Brahmanic culture into India. A smaller group moved west and became the ruling class of the Mittanni kingdom in Syria, where they only had a minimal linguistic influence.

In light of all that:

Could these burial sites at the Andronovo culture technically be precursors to future vedic rituals?

Yes. That exact connection was one of the first major pieces of evidence for connecting the Indo-Iranian languages to Central Asia, and by extension the Indo-European origins to the Yamnaya culture.

The Vedas continuously refer to the Aryans as a certain ethnic/religious group.

Was the Andronovo culture then technically the source of the “Aryans” that were migrating into India and Iran at the time ?

The word "Aryan" has a complex history, even without the Nazis. It does seem to originate with the name for some kind of in-group in the early Indo-Iranian languages, aryaman based on the Vedas and the Iranian/Zoroastrian Avesta. Whether that was supposed to be a particular sect or everyone in the shared culture is not entirely clear. I think I've pretty clearly established the connection between the Andronovo and the Vedic migration into India.

While Aryan is still used as an ethnic identity in parts of India and Nepal, it did not have the same historical ethnic connotation there that it developed in Iran. We can say with certainty that "Aryan" referred to all Iranian peoples as a cultural or linguistic whole in the minds of the Achaemenid Persians by 522BC based on its use in their royal inscriptions. That definition already had a religious connotation, tied to use in the Avesta and that connection only strengthened over time. By 400CE, Aryan had become "Eran" and Eran had been limited to those people who were both Zoroastrian and culturally Iranian. In the Islamic period, the same rough geographic and cultural boundaries stuck but the religious connotation was lost and Eran ultimately became a geographic label, giving us modern "Iran."

The linguistic group that remained in Central Asia developed into the Iranian language family at the tail end of the Andronovo cultural period. By all indications the Scythian language or languages preserved in small bits a pieces by Greco-Roman sources was in the Iranian family. This is also reflected in their clothing and artwork both as seen in actual Scythian archaeology and Greek and Persian depictions.

As that linguistic family was forming, some interesting developments connecting to the Scythians emerged from the Andronovo culture as it fractured (1800-1500BC). To the east, several Andronovo-esque material cultures emerged in southern Siberia as far as east the Altai Mountains. To the west, the Srubnaya culture appeared in the Pontic-Caspian Steppe. Srubnaya is particularly interesting in the context of the Scythians because it represents the first major Yamnaya-descendent material culture to move from east to west. At that point the Scythian culture hadn't emerged yet. In fact, these archaeological cultures in both directions would continue to fracture and change for a few centuries, but the cultural and presumably linguistic movements that eventually created it were in progress.

Around 1000BC, we're getting closer to the Scythians and the evidence for some of that East Asian admixture you mentioned. This is the period when the first horseback nomad cultures start appearing on the steppe. Historical evidence identifies at least one western example as the Cimmerians, but several similar groups emerged in the Pontic-Caspian region around that time. There's not evidence for these early nomads as a single culture, but the cultural and linguistic similarities from their shared Andronovo and Iranian roots helped facilitate cultural contacts with other groups across the whole region.

The best historically documented examples of this come from the c.600CE when Cimmerians formed alliances with the Medes, a more settled Iranian group in the northern Zagros Mountains. However, the same similarities may have assisted cultural exchange with other post-Andronovo groups on the far eastern ends of the steppe. This is the period when DNA evidence from that region would have had the opportunity to enter into the wider steppe community for the first time, something that has actually been confirmed by studies like this by Krzewinska et al. The same study does address how these results should not be taken as gospel or as proof of any other influences.

The Scythians proper emerge around 700BC, first in the Pontic-Caspian region once again, but ultimately went on to form a cultural continuum from Crimea to the Altai Mountains. Unfortunately with genetic evidence inconclusive and linguistic evidence non-existent for much of that region, the "Scythian" identity applied to a lot of archaeological finds only really covers material objects that all seem to belong to groups with similar preferences and beliefs.

We can't identify the Scythians as a monolithic culture. We know from written historical sources from both the Greeks and Persians that many cultural groups existed within this overall region and category we call "Scythian." The Persians mention several groups. Greek sources identify many more. This also came to include groups like the Sarmatians that the Greeks and Romans identified as distinct, but whose purported homeland remained part of that Scythian "cultural continuum."

Did they know the Shakas had the same descendants.

The Persians and other Iranian peoples certainly did. In the "Behistun Inscription, Darius the Great condemns the Saka rebels for not worshipping Ahura Mazda, a sort of religious condemnation seemingly reserved for other "Aryan" groups in Achaemenid philosophy. Whether the Saka themselves agreed with this identity is unknown. Likewise, its very hard to tell how the Indian, settled Iranian, and Scythian components of the old Indo-Iranian group related to one another around that time. The Indian sources are shockingly quiet about anything to the west of Taxila prior the Mauryan Period. Later, when the Indo-Scythian kingdoms developed in the 2nd Century there's no acknowledgement of the relationship.

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I’ve always read that they’re a large heterogeneous group. But after seeing some videos online I'm not sure.

If these are recent videos, then I'd have to guess they're probably referencing a genetic study conducted in the last few years, specifically this from Miller et al. The first version of this study got a lot of media attention for its claims that the Scythians were more homogenous and less nomadic than previously envisioned. The initial publication included data from far outside the actual Scythian period (700BC-100CE), but even this updated version comes to similar conclusions.

However, it's still not a great study to make conclusions about the wider Scythian culture. It's evidence is entirely from Ukraine, at the far western extent of the Scythian range where they had the most direct contact with settled society and are known from historical sources to have settled into static cities and towns. This is potentially compounded by a concentration of samples collected from a Scythian kurgan cemetery near the mouth of the Dnieper River, an area where we would expect to see an even greater concentration of static traders.