Does anyone knows how to disable installing unkown apps on S21 Ultra android 11

انا عاو  اعرف اشغل عرض الاس ٢١ الالترا مع اليوتيوب و ال osn ممكن تي معل مه او افادة

want to buy a new phone a flagship either Note 20 ultra or S21 ultra but I am still not sure which one so I need advise from anyone used both to tell me which one is good

I have samsung galaxy note 4 and I want to know why my battery keep lose charge so quickly even thought I but a brand new original authenticated one and I am not a heavy user only check Facebook and that all I put all my apps on always  save power mo...