If you're a late beginner/early intermediate player and trying to move past the chord shapes and scale patterns rut, I strongly recommend William Leavitt's A Modern Method for Guitar. It's hard - really damned hard - but it's logical, the difficulty curve is fairly linear once you get past the shock of traditional notation, it inculcates good habits from the outset and it provides you with a very broad base of technique that is applicable to all styles of guitar.
https://www.amazon.com/Modern-Method-Guitar-Volumes-Complete...
Oh man, I missed this app when I switched from Android to iOS. So glad it's finally on iOS!
https://www.soundonsound.com/search/articles/%22Synth%20Secr...
Once you've got that down, most popular synthesizers use a very similar "subtractive" architecture, so once you understand the basics of your chosen one, it's simple to follow tutorials using other synthesizers. From there I'd recommend finding a youtuber you like learning from even if they don't use the same synth as you.
I make typical FANG SWE income and I still consider it to be a lot..
It teaches you step by step all the aspects of a synth engine: oscillators, wave shapes, amplitude, filters, modulation, unison, FM… But most of all it trains your ear, which allows you to replicate the sound in your head and build it from scratch.