One of the best resources on the web for learning Photoshop is PHLEARN. Now they have a free 30 day course online, which they say takes you from the basics to more complicated stuff.
I thought it may be useful because I have learnt my Photoshop piecemeal, as I moved from solving one issue to the next. Perhaps, therefore, an overview was overdue.
So if you have resolved to learn more Photoshop in 2020, this is a good start. So get started here.