Aoyama & Midoriya: because of the cheese thingy that happened, Aoyama would sometimes join the dekusquad on some adventures
Iida & Hatsume: after being advertised on the sports festival, Iida would join Hatsume in the lab to help be a 'test subject' for her
Ojiro & Nirengeki Shoda: when they both 'resigned' from the 2nd round in the sports festival, they watched that round together and became friends
Kaminari & Yaoyorozu: doing lots of study groups together, they became best friends along with Jiro
Jiro & Bakugo: around after the school festival, they would practice together, and with Tokoyami, they were the emosquad
Sero & Shoto: probably around second year, todoroki will apologise for what he did in the sports festival on the first year, and they'll hit it off, being the two "calm" people between the bakusquad and dekusquad
Tokoyami & Shoji: they're both "emo bffs" together with Tsuyu sometimes, they hang out in Shoji's room because it's the room with the most space. They even train together, Shoji's like Tokoyami's younger brother
(If you guys have any bestie headcanons, comment below what it is)