Something that took me quite a while to understand, as nothing made this clear at first, is that 'Macedonian' as a separate identity practically disappeared from the Seleucid Empire within about a generation of Seleukos I's death. He is the only Seleucid monarch who was identified explicitly as a Macedonian, both within the Seleucid state (as evidenced in Babylonian texts, among others) and without. This is mirrored by a development among those Greeks who were settled into colonies in the Seleukid Empire- within a generation or so it becomes impossible to tell Thebans, Corinthians etc. apart by anything material or cultural, the only remnant of their specific Greek origins that remains afterwards being personal names. This likewise applies to Macedonians, who become indistinguishable from Greek colonists unless you can identify their line of descent.

The other half of the explanation is 'well how did so many Greeks/Macedonians get to Bactria in the first place then Daeres?'. The simple answer is that, whilst the Seleucids had control over the region, they treated Bactria as an important location for Hellenic settlement, as they did with several other regions of their Empire. The reason why they spent the time and effort refounding temples, founding new colonies, and bringing Hellenic colonists out that far east is because Bactria was valuable to the Empire. Its mines produced (at that time) the world's only supply of lapis lazuli, along with gold and jewels. It was also a highly urbanised region prior to Alexander ever showing up, with a long history of urbanisation dating back to the Bactria-Margiana Archaeological Complex of the Middle Bronze Age. The Oxus, known today as the Amu Darya, was heavily used for agriculture, with long, multiple kilometer long canals in areas given over to intensive agriculture. In other words, it was rich in tax income, in human wealth, and in material resources.

There are also some other considerations for Bactria specifically- at that time, Bactria and Sogdiana tended to be treated as the same entity under successive imperial administrations. Seleukos' son, Antiochus I, was himself the son of Seleukos and Apame, who was either of Bactrian or Sogdian royal descent. Notwithstanding these particular family ties to the region, Antiochus was also co-ruler with Seleukos for several years, and had a special area of responsibility over the satrapies bordering the Eurasian steppe, including Bactria. And the fact that the satrapy bordered the steppe, and nomadic cultures, made it a major military concern of the Empire. The Seleucids settled Greeks in areas that they wanted to gain political and administrative control over, but also in areas where they wanted to guarantee a reliable military presence; they were perfectly prepared to use non-Greek soldiers, but the backbone of their army, in particular their infantry, was always Hellenic, or Hellenic descended at any rate.

So, this led to most urban centers in Bactria becoming heavily settled by Greeks, or new urban centers entirely being created with Greek colonists. This Greek settlement never seems to have penetrated very deeply at all into rural Bactria, but when it comes to ruling over large territories in this time cities were hugely important. As well as representing a fortified location that could be defended, they were also usually the economic and administrative center of the surrounding region. The fact that Greeks, in relatively large numbers, controlled these urban locations, as well as forming the ruling aristocracy of the Seleukid and Greco-Bactrian states, meant that even without any new Hellenic settlement after the 2nd century BC the populations were large and important enough to continue for several centuries afterwards. The fact that Greeks and Greek culture continued to have importance in the region is clear; the Kushan Empire, for whom Bactria was effectively their core territory, oversaw the adaptation of Greek script to write the Bactrian language, the only Iranian language to have ever had Greek script as its written form. They also continued to refer to gods by Greek names, alongside their Iranian and Indic equivalents, along with adopting Greek visual design and employing Greek artisans and architects for their various projects.

As for the ethnic arrangement of Bactria, it's incredibly difficult to tell. I wrote a thesis on this exact question and the evidence is both scant and not very helpful. Without going into an entire other essay-length answer, it seems to have been that religious and administrative authorities from the Achaemenid era continued to have power and patronage, with Aramaic only slowly phasing out as a local administrative language. Local cultural influences certainly made their mark on the Greek colonists, influencing their architecture and visual creations, especially so as time wound on. We can also evidence civil servants and administrators with non-Greek names, though the only real period with this kind of evidence is very late on, only a decade or so before the Greco-Bactrian state was conquered by steppe nomads. But we also see that the cities that were designed were very typically Hellenic, with the exception of religious sites there's very little evidence of consideration for a civic life that wasn't Greek in its seams and forms. We also have some indication that access to the administrative and royal areas of Bactrian cities was tightly controlled, though we cannot prove exactly who was excluded. To really answer this question we would need more legal texts from Bactria to emerge, along with a much wider survey of domestic archaeology than current exists. Almost all conclusions rely on circumstantial evidence with little clarifying context. For me personally, I'd say that the likeliest scenario was that to begin with Bactrians, Persians, and other non-Greeks in Bactria lived parallel civic lives to Greeks, though any truly economically or politically important area would have been monopolised or overseen by Greeks. However, I think that as time ran on, those who culturally identified as Greeks came to have more and more non-Hellenic ancestry, and those without Greek heritage came to partially assimilate. In addition, the local and Greek aristocracy likely merged, not to the point of total fusion but enough to render them near peers. We can definitely say that syncretic religion had blossomed in Greek ruled Bactria by the end of Greek rule, and under the Kushans and other later rulers these fusion religions and cultures only grew closer, with Greek culture and religion remaining a part of that until at least the 4th century AD.