If I enable FileVault on my Mac, then it is stolen and someone removes the drive and attaches it to another computer, are the content of the drive secure? Can they be seen, coped, extracted?
No. File-vault is a whole-drive encryption. What you describe is pretty much the point of using it.
Here's a recently updated Apple article on Filevault. https://support.apple.com/en-us/HT204837
The only caveat I would put to this is that you will want to choose a strong password for your account(s). The encryption key is encrypted by a key that is your password. And that information is all stored in an encrypted plist file on your recovery partition, so, it the drive could be crackable if the password is simple enough for a dictionary crack to get it. And should a thief and or three-letter-government-agency be able to decrypt the plist.