Yeah, you are in taking all that companies new procedures/culture etc, the new people who all already know each other, and it’s a lot but give yourself sometime and don’t psych yourself out. When I would feel impostery I used to lean into my role duties and use them to stiffen my spine.

Had a reply fail, sorry for double post.

First of all, congrats on the new job! Second of all I know this doesn't really help because you know it deep down already, but it bears repeating until you start to accept it:

Everyone. EVERYONE around you also feels like an impostor to some extent. It does get better, but it never really goes away. Totally normal, and I'm sure you'll do great.

Don't worry, I'm a senior developer too and if I can do it, anyone can.

Serious answer though: there's always a lot of learning in the first few months of a job. You can be senior in one job but then walk into another and half the things you knew that made you senior in the previous job are no longer relevant. But that's ok, you just have to be willing to learn.