All my previous phones can't seem to last a whole day before it requires charging. My current phone the Vivo X90 Pro's battery capacity is 4870mAh and it can't last up to 12 hours with moderate to heavy use, and I will need to charge it in the afternoon as the battery power drops to less than 50% with moderate to heavy use.

I was wondering, if looking at the battery spec what is the capacity that one would be looking at for the phone to last at least 12 hours with moderate to heavy use, say 7am to 7pm. Will a phone with 6000mAh+ battery capacity be able to last up to 12 hours with moderate to heavy use?

It's not exactly a 'user problem' but a matter of preference. As we all know, battery capacity is not the only criteria that is important in the selection of phones as there are other more important criteria that take precedence over battery capacity.