How does it work? I know all data is encrypted but when we start computer does it decrypt the data we use on disk ? Or does it decrypt but store in RAM only ?

If you set full disk encryption on a linux install feature, you will have to put a passphase to decrypt the disk in boot.

Firstly there is no way to put everything into RAM. If you select to encrypt the disk your operating system is located, you will have to enter a password in boot as RoyalNavy said. If you select to decrypt an another disk you will have to decrypt it when you want to use it.

In windows the disk encryption process is working like Linux. You can use preinstalled bitlocker feature.