There are plenty of tutorials, you just need to know where to look for them. You could also look for open source projects, I know for sure that there are many XNA/MonoGame Super Mario clones on GitHub and GitLab.
For simple platforming mechanics: Simple platformer game tutorial
Platformer2D example project: MonoGame sample projects
Note that not all XNA/MonoGame tutorials work 1:1 with the version you are using, so be prepared for that.