Can't Decide on a Backend

I'm trying to build an application that will have a web application and also mobile applications (iOS and Android) eventually.

I don't know anything about backends, and I've seen different nocode / low code backend solutions, as well as headless CMS.

Can someone explain the difference between some of these no code backend solutions e.g. BudiBase and headless CMS e.g. GraphCMS ?

Also, can I use any of these as a centralised database that will feed both the web and mobile, or should I just stick with something more traditional like Firebase?

It doesn't have to be the specific examples I gave, but there are so many options, I don't know what to pick.

I'm planning to build the web front end with WeWeb (weweb.io), or possibly Bubble.

Thanks.