At Amazon in particular they pay you the cash value of your stock for the first two years until it vests and you can start actually cashing it out. They have to since they have a maximum wage of ~$170K (varies a little depending on city).
Isn't Amazon famous for clawing back starting bonuses if you don't stay for 4 years or something like that?
I don't think so, because I think the most common tenure is just over 2 years. I know a lot of people who leave right after their second year cliff (the vesting isn't even and most of it comes at the end).
No, the sign-on bonus is prorated monthly and you don’t pay back what you got.
However, you might need to pay back part of relocation money, if you leave before two years mark.