dumb by default

Sometimes software vendors provides their own repository for specific releases. For instance qbittorrent latest releases might not be available on apt database, thus we can use their ppa as an alternative for latest stable releases under ppa:qbittorrent-team/qbittorrent-stable.

Easy way out

software-properties-common provides functionalities to manage independent software vendor sources, or even your own distribution.

To check whether software-properties-common is installed:

1dpkg -L software-properties-common | grep '\bin'

and install by running:

1# check package info
2apt show software-properties-common
3# install just like any other package managed by apt
4sudo apt update
5sudo apt install software-properties-common

which now exposes command apt-add-repository. Passing ppa or repository url will add registry to either /etc/apt/sources.list or as a file inside /etc/apt/sources.list.d

1sudo apt-add-repository ppa:others/managed/repository
2# check if package is registered as apt repo
3cat /etc/apt/sources.list
4ls /etc/apt/sources.list.d

GUI variants also available assuming you have X11 forwarding. which are software-properties-gtk and software-properties-qt.

Manual (as some has mentioned)

To add custom vendor ppa(s) or url edit following files /etc/apt/sources.list which is sometimes not recommended (???). instead create new .list file and add them to /etc/apt/sources.list.d

I have not done this or tested it, so avoid this at all cost.