I have Elementary OS 5.1 Hera, and I would like to use PPAs. That way, I can install some more software not in the default repositories. How can I do this?
Where possible, it's recommended to not use PPAs at all. Adding a PPA is essentially giving the owner of the PPA root (administrator) level access to your system. It is possible for them to maliciously or accidentially replace system packages with broken/unpatched/malicious versions.
PPAs by trusted vendors are probably fine (Ubuntu, Libreoffice, etc), however, always be aware they run the risk of breaking your system if they are misconfigured.
If the software you want is available from flathub.org, then I would recommend getting it from there instead as Flatpak is a much more safe and secure packaging format.
Elementary, like Debian and some other Debian derivatives, doesn't support PPAs out of the box. In order to use them, you'll have to install
Open your terminal with CTRL + ALT + T and simply enter the following:
sudo apt install software-properties-common
Once you've done that, you can add PPAs in the normal way. For example, this is a LibreOffice PPA:
sudo add-apt-repository ppa:libreoffice/ppa