I forget which Linux etc you are using......but if you using any of the Linux Mint stable, it will have qbittorrent in the Software manager
Install the one that says, "Bittorrent client based on libtorrent-rasterbar with a Qt5 GUI.....not the flathub
Open preferences, and go to Downloads....then scroll down to Saving Management.
A little way under that title you will see Default Save Path
mine is : /media/brian/TV & Timeshift/ .......I choose to use a external hard drive to save torrents to.....It would probably be easier if you saved them to downloads....or even the desktop. Your choice.
The path that you type in there must be accurate....otherwise it just will not download and will throw error messages.
To download an .iso....just go to Linux or Ubuntu or wherever, find the torrent link and click it to save it to your desktop or wherever.... ... When you double click on the file on the desktop, it will open a small window asking if you wish to have blah blah open that file(i cant remember the exact name....just tick ok)It will use qbittorrent to open the file and the download will begin., to wherever you have nominated in Saving Management.
Because you are only downloading iso files associated with Linux etc....there is no need to use a VPN....in other words there is no reason to try and hide your IP address from the powers that be.....Why?....because what you are downloading does not break or infringe on copyright and is absolutely legal.
Only when you download movies or tv shows, or sometimes popular books, does the question of copyright etc raise its ugly head. You definitely need a VPN if you do that.