If you want to be skilled admin your best learning resources will be specific documentations which for almost every software exists online.
Interpreting and absorbing documentations is not easy to do, but the more you do it the easier and more natural thing it will become to you.
Once you get used to it, you'll realize documentation is the most reliable source to learn.
Books get out of date, online articles get out of date, tutorials get of of date but documentation does not because it's updated relatively fast compared to books, articles and tutorials or videos.
Only downside with documentation is that it's sometimes poorly written or non existent, but my golden rule is to not use software that lacks solid documentation, this way you deal only with stuff that is well maintained so you don't need to waste your time troubleshooting for hours.