1. The best option is to use Disk Cleanup (cleanmgr.exe) to clean up the %TEMP% folder. However, note that, by default, Disk Cleanup doesn’t delete items that have been accessed in the last 7 days. Make sure to start cleanmgr.exe as administrator so that it can clear the per-machine TEMP files (C:\Windows\TEMP) as well

2
When you select all in the Temp folder, then press delete, all files that can be deleted will be, files or folders that are currently in use will not be deleted, this is normal.

Is there many files/folders in the Temp folder that cannot be deleted, can you please provide a screenshot of the remaining items.

3
Disk Cleanup is the best method to delete all files that are not necessary on your PC that can be safely deleted, you must tun Disk cleanup twice as explained below.

Open File Explorer
Click on your hard drive
On the Manage Tab – Disk Tools, click Cleanup
Let Disk Cleanup do its initial scan
Select Everything in the list and choose Delete

Start Disk Cleanup again
Let Disk Cleanup do its initial scan
Important – this time in the resulting dialog, click ‘Cleanup System Files’
Let Disk Cleanup run another scan
Select Everything in the list and choose Delete