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The system Temp folder (you can find it by typing its path in the File Explorer search bar) is basically a graveyard of files that Windows forgot to delete - you can safely clear almost everything in there! Just close all your programs and browsers first, or the system will complain that the files are in use.
Programs really do help better here, but if you want to avoid them - you can manually delete temporary files through the AppData folder, clear your browser caches and trash, and search for large old files using size-based search. You should also uninstall programs you don't use through "Settings" - that frees up a noticeable amount of space. But honestly, doing a thorough cleanup without specialized software is tough, because a lot of junk is hidden in system folders and you might accidentally delete something important.
Don't forget about the Downloads folder - that's where dead software and installers tend to pile up, and you can safely delete them after installation.
Actually, cleaning your disk manually is time-consuming and incomplete, but if you're set on doing it without programs, the key isn't in folders but in the largest files. I used to struggle trying to figure out what was taking up space until I discovered I'd accumulated five gigabytes of webcam recordings buried in Documents - they were just sitting there and I forgot about them. So first use the built-in analysis: open the properties for disk C, and at the bottom you can run the storage tool (on newer Windows it's Settings → System → Storage → analyze disk), and it'll show you the heaviest folders by category.
After that, dig into specific locations - the same AppData, Download, Temp that were mentioned above, but not blindly, knowing how much space they actually take up. Most of the time the main "junk" is old Windows restore points and a huge hiberfil.sys file (if you're not using sleep mode, you can disable it), not some minor caches.
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