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It depends on the mods. Many add their own features to world generation, such as new ores or trees, and have to be enabled in your world prior to world generation. Others add new recipes or other functionality which does not depend on new worldgen features, so you can enable them after worldgen.

Once mods are installed, you can enable and disable them on a world-by-world basis using the Mod menu when selecting a world under New Game. All mods are disabled by default, so don't forget to enable the ones you want after creating a world and before playing in it for the first time – it is entering the world that actually starts up the world generation!

Hope this is helpful!

Definitely the piece of information I've been looking for. The Content Database of Minetest official mods is so vast; I spent a whole evening yesterday just familiarizing myself with it. It is going to take a few more evenings to figure out which ones I'd like to turn on for sure. I'm going to google for their discord right now -- it feels like the questions will keep popping up! Cheers!

That is great news! Best of luck to you, and have fun with the block game :)