Topic: Buying Game using Account or CD key

I understand buying a game using an Account ( username and password supplied) method or CD key
is cheaper than buying directky from Xbox.
Are there any limitations using these 2 methods compared with buying from Xbox/MS?
I haven't bought a game yet by either method.
I have a Xbox One Series S and dont bother with rewards/ achievements.
My console does not have a cd drive - does the cd key method not require one or is is just an electronic key?
I wont use multiplayer and dont have any gamepass.
Thankyou for any help

1. Xbox on some occasions have games that are cheaper on the store than on a website.

2. The only limitation that I can think of is region/location.

3. CD Key method is an electronic key, no disc required.

4. Is there a reason why you don't have Game Pass?