Or if you built your base over a yeti spawn and have an area inside of it without foundations it is possible for it to spawn in the part with no foundation (foundations move and/or eliminate spawns when built on)

Maybe I read this too literally. It is possible for them to go through the door if they get just the right angle, but they can't actually open them.

there is just me and my friend auth and all base claimed, admin came and checked and it was fine.
but i woke up and a yeti walked through and started pounding me with his fist opening doors with his vision

The AI can not open doors. Either someone left it opened or maybe you had part of your base on another plot w/o an ownership stake on the extra plot... meaining anyone can open the doors there.