Do all devs know how to write these Goolgle/Amazonesq algorithms off the top of their heads? ex: build singly linked list from scratch / pre or post binary tree traversals etc..

That is first or second year computer science. Fundamental data structures.

Yes but I'll try to elaborate. They do for interviews at least. Knowing how to build a single linked list is something that should be taught in any Object Oriented Programming class. Employers will expect you to know them for interviews because there are not a lot of good ways to evaluate how good someone is at *thinking*.

Also I want to add that a job interview should be looked at as a final exam. You study for exams so why not for interviews?

FWIW a hiring manager that knows what they are doing will not ask you trivia and there is no way you can prepare for their interview questions.

I don't know how to build linked list but I do know how many distinct words I can make out of the phrase "peanut butter". Kidding I don't know how to do that either. That was a real question at a recent job interview.