Set up Linux on your computer or laptop or whatever. Tell them that you need it for computer science class or IT or whatever the hell.

Then your parents likely won't know how to use it enough to find your personal files.

Also, you can do cool tricks like hiding your personal files in a place where nobody would think to look, like ~/.local/firefox or something like that, and you could also do some tricks with permissions to make it harder to find.

Something I used to do was make a folder like ~/.local/firefox/folderWithYourStuff/
and I would remove read permission for the 'firefox' folder, but 'folderWithYourStuff' would still have read permission enabled. So whenever you want to look at your files (like your ecchi pictures and hentai and stuff) you would just type ~/.local/firefox/folderWithYourStuff into the 'location' bar of your file manager, and there it is.