This is precisely what memory is for. Memory stores the processes and services you use regularly to allow them to be retrieved quickly when you need them. It's very normal to have over 1.5 used. It's lower when you reboot as those processes haven't run yet buy as soon as they do, they'll fill memory.
When you're memory is close to "full", Android starts killing the processes to maintain optimum levels.
If your memory is empty , it's useless. There's nothing wrong here
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