Its only the really big OEM holders (like Dell & lenovo) that have the licenses that restrict their reuse in other PC. Almost all PC converted from 7 & 8 to 10 can have the license themselves moved to another PC. It is one of the benefits of the upgrade.
there is very little difference between retail and OEM. Only one I could see is how many times it can be moved. retail is likely to let you move it more than once.
These days the Windows 10 license is tied to your MS account, not the PC, which makes it much more likely it will transfer to an upgrade(d) computer.
Not all. It really depends when Win 10 was installed as to if it is linked to an account or not. For a long time Microsoft didn't make it difficult to create a local user on PC, at installation stage, and if you had swapped from 7 to 10, you may never have linked PC to your email address. My PC was like that until January last year when I last clean installed and mistakenly linked my MSA to licence.
So I can't tell you how many are still linked to hardware but it isn't none.