why is "na" used and not "w"? I believe "u" isn't used because that is only when you are at a person's place i.e. "u babci"
But it seems na, w and u can all be used for "at" in different contexts. It's hard to decipher
Yes, "u X" is generally "at X's place", so this one is rather clear.
Events rather take "na".
Places mostly take "w" for being inside a building and "na" for more open spaces, but there are exceptions to that and they aren't even that rare.
Is there a differentiator between "the conference" and "a conference" or is that all contextual?
Not really, we just don't think in terms of a/the, so the a/the distinction is something confusing to us. Whether the conference was mentioned before or not... we don't care. They're at some conference.
I think, "in the conference" should also be accepted, as well as, "at the conference"
In American English it sounds really weird. I've lived here for over thirty years and have never heard it, neither have my three relatives, who work in the academia and often actively participate in organizing conferences.