Usually RSUs on joining. Not sure on size. During the annual review you may get a smaller RSU refresh or more if getting a promotion. At the end of the year there are often larger rewards given for high performers. They used to have a department level allocation to spread around as management desired. Not sure if it's still that way.
Does Annual review mean during MBO?
MBO is bonus time in the spring. Annual review is the summer perf review.
Not to everyone. I think something like 10-20% of employees get a refresh grant in any given year.
What about stock during joining.. for senior engineer/ staff engineer
I've heard of senior engineers getting 1000-1500 RSUs over 4 years at joining. I guess for staff would be like 1800-2000 but that's just a guess.