You can install Minecraft Forge for free from the Forge website, and then use it to download and enable mods.

Here's how to install Minecraft Forge, use it to optimize your mod library, as well as some handy recommendations of mods to try out when using Forge.

For Iris Shaders (for modern visuals and fabric users), go to the Iris Shaders page— https://irisshaders.net, download the Fabric version for 1.21.5, and then install it using the Fabric mod loader.