It's fairly simple. You have a "Category" dropdown that you can use if you want. And if/when you select it you can also narrow down your search parameter by clicking on different tags that are bellow that dropdown. You can use multiple tags. The search bar is not precise so you can just find a mod by a keyword if you don't remember it's full name.
It's browsable on your phone but not really great there. Usable but not amazing as I couldn't find a quick solutions to Google Sites not allowing me to properly resize stuff.
Here are the different categories and tags bellow with a simple explanation:
Categories
Gameplay - These mods either add a new system or makes changes to an existing one. Simply put, a system is a gameplay loop. Religion, colonization, mage & artificers etc.
Flavour - Anything these mods add doesn't affect the gameplay at all.
Miscellaneous - Mods that don't fit the other categories.
Tags
Performance - Lowers the performance impact of Anbennar or EU4 in general.
Monuments - Adds in new or re-works the older monuments/great projects.
Map - Changes the layout of the map, whether it's by adding new provinces or remaking the nations setup. Does not include adding new nations without changing the ones already present in the mod.
Missions - Adds new/re-makes old mission trees.
Nations - Adds a new playable nation(s) to the game. These mods often add a bunch of new content for those nations, but not always.
Religions - Adds a new religion or makes changes to an old one.
Compilation - A single mod that aggregates multiple other sub-mods, with or without adding changes on their own.
Tweaks - A change, or several of them, that affect/do a specific thing. Whether it's a changed mission tree requirement or a decision to allow you to embrace a previously tedious to acquire religion.
War - Sub-mods focusing on war in general. Changing how combat works, adding a new CB etc.
Ideas - Changes or adds to the idea groups.
Buildings - New or rebalanced buildings.
Trade - Changes to the trading system. Most often trade nodes.
TradeGoods - Any changes that affect the trade goods. Adding new or rebalancing the old.
AI - Sub-mods that affect the AI.
Ages - Changes to the Ages & Splendor system.
Estates - Adding new estates or expanding the current ones.
Subjects - Mods that affect how you deal with subjects.
Development - Changes to the Development system.
QoL- Making stuff less tedious.
Cheats - Can be QoL in some cases or just for when you don't want to go through the Serpent's rot for the 10th time.
Compatch - When you want to use multiple mods together that are normally not compatible.
Systems - Introduces a whole new system or systems (gameplay loop) into the game.
Flavour - Fluff additions to the game that aren't graphical in nature or change the UI
Graphics - Anything that changes the map, units, or other non-UI elements of the game in a graphical way.
UI - Anything that primarily changes the UI