The one thing I don't understand about music theory

I’m in the same situation as well. I really only know how to solo in pentatonic scales and don’t understand much about keys and modes.

My understanding of modes are, just scales starting in a different note of that scale. Fo example: The C Major Scale.
The notes are C D E F G A B. This is called the Ionian mode of the C Major Scale. AKA as just the regular C Major scale starting in C.
But what if we start that scale on A instead of C of the C major scale? It’ll go like A B C D E F G. Which is going to be called the Aeolian mode of the C Major scale. Its just the C Major scale starting on the A instead.
This is just MY understanding pf of it, I’m not 100% sure

The best way to really understand the modes is to play them all in the same key center, so we’ll say C major, the you can compare the differences in sound with the other modes. The formulas for the modes are
Ionian(Major)- 1 2 3 4 5 6 7
Dorian- 1 2 b3 4 5 6 b7
Phrygian- 1 b2 b3 4 5 b6 b7
Lydian- 1 2 3 #4 5 6 7
Mixolydian- 1 2 3 4 5 6 b7
Aeolian(Minor)- 1 2 b3 4 5 b6 b7
Locrian- 1 b2 b3 4 b5 b6 b7
So basically to hear the differences, play a C major scale(CDEFGABC) anywhere on the neck. And then when you want to change the scale to a different mode, make the changes shown in the formulas above^. So to go from C Major to C Lydian you just change the 4 to a #4, raise the 4th up a half step so its C D E F# G A B C. Now you’re playing lydian! Or if you see a “b” like in mixolydian(1 2 3 4 5 6 b7), just lower the 7th note of the scale by a half step, C D E F G A Bb C.
Its as easy as that! This will really help you HEAR the differences as opposed to just SEEING the differences in moving patterns around and someone telling you that the scale you are playing is Lydian or something.
Modes aren’t super important though, the important thing is to hear the intervals and to learn how all 12 notes sound over a major, minor, and dominant chord(The 3 basic qualities of any given harmony). So like a C7#5(C E G# Bb) or Cm6(C Eb G A), and things like that. The harmony and the flavor that each note has and really understand the SOUND of all the different notes is the important part.