Hey there
You have choice.
You don't HAVE to sign into a Microsoft account, but you can if you choose to. Most of the world lives on Microsoft so that is why it is offered. Many users also like to use OneDrive.
Google have their equivalents of.Microsoft products and you are free to use them. Me? I use Google One and back up to Google.
However I have also signed into my free Microsoft account for Excel, Word etc.
I am not employed by Samsung nor do I receive any remuneration as a Samsung Members Star.
My device is an S25 Ultra.