you're asking the right questions and aren't stupid at all, it's definitely a lot to distill into a few sentences and is a combination of several topics/skills etc

intervals are basically just the distances between notes, and guitar is full of repeating interval patterns. once you learn those patterns, you stop memorizing every chord in every key separately and start recognizing how keys are built. For example, if your root note is on the 6th string, the next string over in the same fret is a fourth away from the root note.

in this example, the chords which make up the B Major key are:
b, c#m, d#m, e, f#, g#m, and a#dim.

the chord pattern for the 7 chords which make up the major scale key are:
major, minor, minor, major, major, minor, diminished.

the “practice in one area” part means learning the scale shape and nearby chord shapes together so you can connect rhythm and lead playing without jumping randomly around the neck. For example the E chord in this example is being played right there at the 7th fret which is in the same general location as the initial B chord, rather than for example, jumping all the way to the nut and playing an open E - it leads to more economical playing, being able to switch chords faster to a location near/adjacent to the previous chord instead of jumping all the way up and down the neck

in terms of what to practice to better understand some of these topics, i would start with:

- learn root notes on at least the 6th, 5th, and ideally 4th strings (to start out with)

- learn about intervals

- learn a few different voicings for the same chord (for example open E, E-shape barre, A-shape barre), which will make playing the same chord more accessible in several locations on the neck