Microsoft crm dynamics legacy web interface app

I'm very new to Microsoft CRM, so please fix me if i'm wrong.

I have a business app that has the old web interface. I don't want to setup a new solution for a client and provide a new api. A want to reuse what is already there and adjust what data I will get when I request some project query.

My problem is that I cannot find where the schema is defined. Can someone point me where to look for it?

Right now the api is working and I can retrieve data without problems, but there are some missing fields that are present CRM and not accessible via api. And my goal is to provide those fields

Thanks

Solution

go to make.powerapps.com
on right hand side select your env (dev/Test/Prod)
after that select solution on left hand side
then scroll down complete list you will find default solution
once you visit default solution, It hold almost entire system schema and so on. For example tables, it's fields and so on.

there are many other ways, but this should be a good starting point. Note: It is not recommended to make changes directly under default solution, In fact it is bad way of making changes.