Diff elo algorithms

My rapid rating on here is like 750, but on chess.com it’s like high 300s low 400s. I play the exact same, do they calculate elo differently?

Yes. Lichess uses Glicko-2 and starts at 1500. Chess.com uses Glicko and you can start at 800, 1000, 1200, 1400, 1600 or 1800.

Also worth noting that ratings are relative to other players in the same pool. Lichess has its own unique pool of players so, amongst other reasons as well, you can't directly compare its ratings to other sites / organizations.