That's fine. You can always delete personal files and programs that you don't need if it gets too full. Usually takes a long time unless you have a small 32GB or 64GB emmc C-drive.

Very normal. This is called 'caching' and 'cookies'. You don't download anything. The websites you visit or the programs you run do this automatically so that  passwords are temporarily stored on the disk so you don't have to keep entering them, or your location is identified and stored on the disk, etc. When you exit the  website browser some of this information is automatically erased from the disk. Some of it is stored on the for the next time you browse This is why you should be careful on the type of websites you visit. There is nothing you can do to stop this except by disconnecting your laptop from the internet.

What web browser do you use? FireFox? Chrome? Edge? There are settings in the browser to erase cookies and history data. This is usually not a good idea though. Why do you feel that you need to erase this information? You have a large C : \ drive that will take many years to fill up with ordinary use. Jack E/NJ

Still normal. Every Win10 update adds, removes things on C : \. Major Win10 updates that happen about every 6 mos may add or remove even more. I suggest that you stop monitoring and fretting over free space on your drive. New HDDs and SSDs have gotten such high capacities in recent years that changes in free space can vary by many GBs in a short period of time is very common. Remember, one GB represents only about one or two tenths of one percent of the total capacity of these new drives.