In order to access these doors you will need an evil or atropian key to open these doors to the main boss in the biome area.
You will find the key at one point during your exploration of the game's regions, which will also allow you to obtain weapons and upgrades.
Each red door holds its own valuables, so you will need to collect several keys, one for each door. The good news is that after your rebirth, the red doors do not need another key to unlock.
Almost all arenas have a weapon to pick up before entering, and most of them have a clear barrier requirement gear, so if you don't want to access the boss battle, don't go beyond the weapon, come back when you're clever.
We hope that this guide on how to open locked red doors in Returnal, is as useful as possible and you can get all the keys you need for each door you find.