As you can see with dnf repolist, only the core repository and the update repository are activated on Mageia 7 by default.
The repositories nonfree and tainted are pre-configured in configuration files within /etc/yum.repos.d/ but disabled by default. You can easily activate them by executing the following command:
# dnf config-manager --set-enabled mageia-x86_64-nonfree updates-x86_64-nonfree mageia-x86_64-tainted updates-x86_64-tainted
That’s it. Now, when you try to install a package or upgrade the system, the nonfree and the tainted repository are considered as well.