There is invariably no single “right” way for every situation.
What about having “user” and “relationship” ? All users have a record in “user” and each relationship document is simply a pair of user IDs and the “type” of relationship?
Even if you’re only modelling one relationship (i.e. friends) then the relationship can automatically be bi-directional (depending on how you store the two IDs and how you query). If you were to store IDs like your option 1, then you could easily end up with A being friends with B, but B not being friends with A.
But this all basic database design and there are a lot better resources out there discussing this I’m sure.