My favorite, the most comprehensive course I have seen this this one: https://professionallinuxusersgroup.github.io/course-books/lac/syllabus/
Most knowledge is transferable to most distros. Some distros deviate by using alternate init systems (the part responsible for booting the system and managing services), but most distros use the same underlying suite of software to manage it.
On diference between distros is the package manager, but although the commands may be different, the underlying operations of installing, searching for, and removing software packages are all the same.