You need to manually download mods from a site like
Nexus and extract them in your Module directory [C:\Program Files (x86)\Steam\steamapps\common\Mount & Blade II Bannerlord\Modules]. After this, you should be able to enable them from the launcher.
Some modules have different instructions which should be detailed on their download pages. I would suggest backing up any files that are replaced by mods that don't use the module system.
A small disclaimer, use mods at your own risk.