Here's an answer I wrote about Odr and Oðinn a couple years ago. I touch on Frigg and Freya a bit at the end as well.

Something allude to at the end of that answer but don't really say explicitly is that the Norse religion was not a cohesive whole. We have Snorri and the Prose Edda and Gesta Danorum which all give us hints at how some of the beliefs and stories may have gone. They are not the end all, be all of what Norse belief actually entailed though. So in our main sources that survive they are all separate figures, but in actual practice before Christianity became the dominant faith perhaps they were actually one and the same. It is impossible to know for sure. All we can do is make suppositions off the archaeological and literary evidence we have. Sadly, I don't know enough about it to talk fully about the archaeology but my understanding is that some of the discussion of Freya/Frigg is partially based on the similarity of how they are depicted in carvings.

Please note there is definitely room for other interpretations of the evidence. There is a reason that some very learned scholars have argued for them being the same. The best thing you can do is look at the arguments of each side and decide which you find more convincing.