Downloading Mods
Make sure you know how to Create an Instance before attempting to download mods.
Inside the Edit menu, select the Version tab on the left sidebar. From here, you will need to decide on which mod loader you would like to use. Fabric is available for all versions starting with 1.14. If you are using Forge mods, for Minecraft versions 1.20.1 and below, select Forge; for versions above 1.20.1, it's generally recommended to use NeoForge instead. Content-heavy mods are typically made for Forge or NeoForge, while Fabric mods focus more on performance and QOL features.
Once you have picked out your ideal mod loader, go to the Mods tab, and select the Download Mods option that's to the right. From here, you can choose to install from both Modrinth and CurseForge services. Modrinth focuses more towards free and open-source mods and is generally recommended, however, some older mods may only exist on CurseForge.
It is worth noting that many mods will be available on both platforms.
After you've chosen your mod provider, you can search or browse for any desired mods. Click the Select mod for download button to add each mod to your download queue. Once finished, you can now press OK, and your mods should begin downloading.
Fabric
If you have elected to use the Fabric mod loader, then please ensure that the latest version of the Fabric API mod available for your game version is installed. It is mostly required for Fabric mods.
If it is missing, you can install it from Edit > Mods > Download mods like any other mod.
Quilt
For installing Quilt in Prism Launcher, follow the quilt wiki.