Minecraft Mods don't appear in game?

I have the latest version of forge installed (1.13.2) and i'm trying to download mods. I'm downloading them from the forge mods website, and all the latest mods are 1.12.2, but i've seen a video of someone using them with 1.13.2 forge and it working. When i place them in my mods folder, they are .jar files, but when I startup minecraft it says "error _____is not a valid mod file."

That error appears when the mod isn't compatible with the version of forge you have installed.

Are you sure that you're using the same version of forge and the mod as the person in the video? Different sites will have different versions of the mod, the best place to get mods is from