Every non-gaming-focused Linux distribution offers a checkbox for encryption during the OS install process. Many of the gaming-oriented ones like Bazzite do too.
When checked it uses cryptsetup and the full disk is encrypted.
It's offered, and I think for quite some time already, but it's not default. So it's not wrong to assume that most installations are not encrypted. I think.
This is a easy one.
Both ios and android does the same thing -- the filesystem is always encrypted, factory reset discards the decryption key.
On macOS and windows, that's encryption by default. (yes, bitlocker is the default now)
This is pretty standard nowadays.
Linux, otoh, don't usually do the encrpytion.