Fast Battery Drainage problem

I have an oppo Reno 6 5g phone used it for about a year & recently i faced a fast battery drainage problem & i have changed to a brand new battery from oppo company & i used for one mont normally and the problem persists after a month. May be i have downloaded premium apps from other sources than Google store & but i have unstalled all the apps and the problem still persisting to happen. Thus, what is the problem of this fast battery drainage and how should i fix it?