a power chord uses only a root and a 5th. a root 3rd 5th would make it a major triad.

This.  The whole point of a power chord is that has no sonority (there are no major or minor tones whether they are seconds, thirds, sixths, sevenths whatever) and just the "perfect" fifth/fourth depending on your inversion.  It isn't about sounding "dirtier" but it's more about the guitar line being a melody with a fixed harmony and not actually a series of chords.  It's actually a pretty interesting concept and you could relate this relationship to all sorts of ideas in music both modern and contemporary.

playing barre chords with my thumb

I am really sorry man but that is a horrible idea and you are wrong.  That said, I won't try to convince you because you are pretty set in your ways.  But seriously man you might be risking injury or something, and if not that, there will be a point that will come up pretty soon when you'll realize that your bad technique has made advancement in the instrument possible.  Right now you say I just want to play rhythm guitar dudes!! but you have years and years of guitar ahead of you and it's better to start playing the right way in the first place then retroactively relearn your technique down the line.

So back to the topic:

Yeah there are a bunch of Yesterday lessons on youtube and it seems like a simple enough song

I've learned G, F-minor, and B7 hopefully I'll learn the song by the end of the week if I get off my ass and practice.

I also found this cool site where I can look up any chord and it'll come up on some picture with dots on the frets and strings I need to find which is really helpful cus I get confused.

I know that they are tough early on because your muscles aren't developed (and you might want to use your thumb instead lol) but it will be *really* helpful for you to try and learn barre chords before anything else.  Memorizing open chord shapes is great and there are a lot of songs you can play that way, but it's not a very logical process.  If you learn the barre chord shapes for a major chord, a minor chord, and major/minor chords with sevenths, you'll have a much easier time down the road.  This is because barre chord shapes can just move up and down the neck, so you can play chords on the fly without thinking (it also makes it no trouble at all to play a chord like C# minor major-seventh major-ninth whereas I am pretty sure it is impossible to play that particular chord with open chords).  You only need to know four or five chord shapes to play anything with barre chords, whereas with open chords you probably need to memorize dozens of chord shapes before you can play a truly useful number of different chords.

I am really sorry man but that is a horrible idea and you are wrong.  That said, I won't try to convince you because you are pretty set in your ways.  But seriously man you might be risking injury or something, and if not that, there will be a point that will come up pretty soon when you'll realize that your bad technique has made advancement in the instrument possible.  Right now you say I just want to play rhythm guitar dudes!! but you have years and years of guitar ahead of you and it's better to start playing the right way in the first place then retroactively relearn your technique down the line.

playing barre chords with your thumb in no way risks injury, nor does it prevent you from playing them the correct way when the situation arises!

there is no wrong or right way to play guitar, christ.

youre absolutely right about gain filled chords not being as "clean" (duh). Though whether thats good or bad is completely subjective. As far as clean, full chords sounding "deeper" i must disagree. Generally a power chord uses a root and a 3rd or 5th--cutting out the higher notes. so if anything a clean full chord will be brighter and have more "shimmer".

I know very little chord theory (never cared to learn yet) but even I know this is just crap. Play a C power chord, now play an open C. Notice how the open C has bass and treble? I don't understand how you think cutting out the higher notes would add more shimmer- quite the opposite as it'd be far duller.

I think you misunderstood my use of the term 'deeper'. I did not mean more bass.