1.If you have the original launcher (and a proper Mojang/Microsoft account which you can use for MultiMC and third party launchers if you don't like Mojang's offering, I'm just saying you have flexibility if you don't like it, but it's a fine option, just third party ones can be useful the more you get into modding for multiple versions and making modpacks but if you just want a simple setup that's fine) then you just need a modloader.
2.Modloaders add the modded layer to the game as the game itself don't have support for them. So it's not a Steam Workshop like situation, you have to search for them.
Forge and Fabric are the two current options. Though there have been others in the past but if your just going with say 1.16.5 then those are the two you will come across.
Forge has most mods and has been around the longest but Fabric has it's place and is worth mentioning, I see benefits in both of them so take a look at the mods for either and see which suits what your after more (I'm not going to ignore Fabric like TileEntity will as you may come across mods for Fabric only or both modloaders and need to know this stuff as it doesn't happen for other games. Some mods aren't compatible with the other modloaders and it can be confusing. I know two modloaders is a bit confusing but it's been a thing since Minecraft was in Beta in 2011 so I think it's relevant to mention it to help you out).
That and I've a fan of both, unlike many in the community so I will give fair points for either rather than ignore it.
The major thing to know is that Fabric and Forge mods are not compatible so you have to use either both for separate experiences or stick with one if you find it a lot of effort, each offer similar or widely different experiences depending on the mods so make sure to look for a Fabric icon on Curseforge pages, some do have the Forge logo but barely have started using it so assume most that don't have it are for Forge or the description itself says 'Forge Only' or 'Requires Fabric API'.
3.Curseforge is the safest source to find mods, but Modrinth is a good alternative. Some sites can be dodgey so best to avoid any that aren't Curseforge, Modrinth or Planet Minecraft. So the ones like your 9Minecraft, 6Minecraft and others for example that can appear in search results.