Joining FAANG as senior eng, should I learn the tech stack before I join?

Which FAANG are you joining?

I don't think a week will make a huge difference in your performance. I'd take it easy so you come in excited and ready to work hard! Realize that Big Tech cos will often have many abstractions built on top of the language, which you won't know about until you join the company and have access to the codebase.

If you're going to do anything, try solving some simple programming challenges (Leetcode) in Go. Don't just read a book about it.

I highly recommend going through the onboarding course!! The Complete Onboarding Guide For Software Engineers

I would relax. 1 week really won't make a difference, and given that Amazon is so massive, all their Go infrastructure is probably super custom anyways (making returns on outside studying with generic resources super low ROI).

If you do want to prepare prior to joining the company, I recommend this: "What's the best way to prepare 'before' joining the next team/company?"

Congrats on getting into Amazon at senior! The senior promotion at Amazon is insanely difficult, so I'm glad you don't need to go through that.