Ok, let’s take just one chord:
In C:
G-D / G-B-D a simple ‘G’ major chord in its root position, right? Right. First, you HAVE to learn how to recognize this chord in its various inversions
(we’ll stick with the RH for now to illustrate the inversions just remember that what ever you can do in the right hand you can do in the left):
G-D / B-D-G now, the chord is in 1st inversion
G-D / D-G-B now, the chord is in 2nd inversion
How do you know what inversion to use? It depends on your melody or what YOU want for the melody.
BUT, what if we just play with and add some notes to this simple chord?
Let’s lower the third:
G / G-Bb-D now, we have a simple ‘G’ minor chord in its root position. I’ll trust that you’ll copy this and go through the inversions yourself so that I can simplify this post.
Let’s add the 7th:
G-D / G-B-D-F# we have a major 7th chord (You have to know your scales in order to understand this concept)
Let’s lower the 7th:
G / G-B-D-F now, we have a dominant 7th chord (or, it’s just called a 7th chord)
Let’s lower the third again:
G / G-Bb-D-F now, we have a ‘G’ minor 7
Let’s lower the third and the fifth:
G / G-Bb-Db-F now, we have a ‘G’ half-diminished 7th
When you alter or add note in a chord, you change the color (or sound) of it. Depending on what you want, you can come up with very interesting chords.
I’ll leave you with that, for now. Questions?
And, of course, if I'm missing something in this explanation, knowing that there's more, obviously, please assist, folks.
Again, I think THIS is the kind of stuff that folks have really been seeking when stuck.