Modes are crazy things that are modes, but they're also scales, but they're also just parts of scales or a scale starting from another position. We all get that.
Major scale - Major minor minor maj maj minor b5b7
Or. 1 Ionian relates to a major chord, 2 Dorian relates to a minor chord, 3 Phrygian to a minor cord, 4 Lydian is maj7, 5 Mixolydian is Dominant, 6 Aeolian is minor7, 7 Locrian is half diminished aka b5b7.
While we like to stick to chord tones often with bass.. scale tones as passing notes that fit the above aren't necessarily a horrible bet afaic. If someone calls out "2 5 1 in C" you know it's going to be Dm or Dm7, G7, and Cmaj7. The chord tones we use are intervals within the modes of the major scale. 2 5 1 can also be "II Dorian, V Mixolydian, I Ionian" or.. if you wanted to spice it up you could use a mode from another scale or something else that fits the relative chord.. i.e. pentatonic major in place of the one.
Chords, scales, modes are all relative and leave room for variation. You could spend a life time discovering new ways to put them together, never be the first to have done so but still be able to call it your own.
One of my favorite practical uses of "Modes" is actually mixing modes from two completely different scales to make some passages more interesting.
If I'm playing a pentatonic minor or blues scale, in Am, that's only a few notes shy of A natural minor.. A natural minor is not far off from A harmonic minor. It's a safe bet that I can intermingle notes from all three of these scales, or the first mode of these scales to good effect.
In practice.. it ends up being a fill with a chromatic feel, or one run might sound like one scale and the next sound like a different but not out of place. What I'm really doing is pulling notes from various modes from various scales to create something that sounds a little more interesting than just sticking to the lines on the road.
It's not hard for me to compare this to say a typical one or two chord Funk rhythm on a guitar. The first reaction is "one chord, how boring" but.. with just some movement from your pinky and middle finger, it turns out there are many voicings to any given chord that might fit well within the context of a song. By the time you've spiced it up with some gaps and spaces in the rhythm and figured out all the various voicings of that chord that sound good.. at least to me, it's suddenly sounds much much more interesting and becomes lot more fun to play.
I mean part of why we learn them is to recognize how most music we listen to is pieced together from familiar shapes/patterns/chord progressions. If you play and listen to the modes enough you will start to hear them in every song you listen to. You get to a point where you can react to those sounds immediately.. and thus you've begun to learn how to improvise on your instrument.
Improvisation is the other part of why we learn them, and the one that stuck with me the most, is so we can take them can get creative with them and add flavor to your own musical creations. Each chord you play has its default mode in relation to the key of a song, OR.. each chord fits with multiple modes from multiple scales. Figuring out what works and doesn't there can be fun, and also sometimes results in something I like to call composition.
Don't be afraid of experimenting with these guys. I mean.. I can't read sheet music and I'm jealous of those of you that can, but scales, chords, modes intervals and rhythmic values.. these are the keys of creativity and why I personally love music. Don't be afraid to sit down and explore them and sound bad doing so as you learn what sounds good. And remember, the audible aspect will probably make more sense than the written aspect depending on how developed your ear is.