Have you closed all running apps? (Most apps carrying on running, and using resources, unless you actively close them, rather than just ceasing to use them).
[Press the three lines and then 'Close all']
Have you closed all unused browser tabs? (Unlike in a PC browser, where switching from one page to another automatically closes the first page, going to different websites in Android browsers leaves older ones open in hidden tabs, using a lot of resources).
[In Samsung Internet browser, press the rectangle with a number in it near to the bottom right of your screen. Then press the close 'X' button at the top right of each tab in turn. Google Chrome works in the same way, except that the rectangle with a number in it appears at the top right of your screen].