When I want to download a file via Tor browser desktop, the file picker window shows a Downloads directory inside the browser’s directory as default and I have to move to the user’s home manually to save the file inside the main Downloads directory. Why there is a Downloads directory inside tor browser’s directory and why it is default?`
You never mentioned what OS you are using. I just downloaded a file and it went to my default user download directory.
I am using archlinux. Default path when I wanna download a file with tor browser is /home/user/Downloads/tor-browser-linux-x86_64-14.0/tor-browser/Browser/Downloads not /home/user/Downloads.
Hmmm! Then I can’t help. Guess you will need to wait for a linux user to reply. I use Tor on windows. I have a version on Ubuntu but never use it.
Default Downloads directory, on Linux at least, is the one in the Tor Bundle you identified. You can set another one in General settings:
Same on Windows 
I know that and it is set to /home/user/Downloads in my case. My main question is that “Why there is a Downloads directory inside tor browser’s directory?”. Why do we need it?