More great advice here:
For scale, riff and melody playing:
https://youtu.be/GaQSLpBnRCs?t=38
Thumb position:
https://youtu.be/GaQSLpBnRCs?t=98
Generally an excellent, detailed explanation of the ideal finger placements.
Here's the classical perspective:
https://www.youtube.com/watch?v=2TyXvcc-UQc (again, scale-playing position, thumb position, with some basic exercises.)
For barre chords, here's some advice and pics:
https://andylemaire.com/bar-chord-hand-positions/