Fedora & SailfishOS: drop .pem files of root certificates here and sudo update-ca-trust.
.pem
sudo update-ca-trust
Debian & co: drop .crt to /usr/local/share/ca-certificates/extra/ (create if doesn’t exist, /extra/ may be anything) and run sudo update-ca-certificates
.crt
/usr/local/share/ca-certificates/extra/
/extra/
sudo update-ca-certificates