Ah yes. The 7 chord dilemma. It's confusing as hell, so you're following the same path as everyone else when it comes to them. lol Seven chords are so confusing because there are so many different types of 7 chords and the notation of each doesn't really help alleviate any confusion. So start with learning the difference between dominant 7 chords (ex. C7), major seventh chords (Cmaj7) and minor seventh chords (Cmin7). This may help you moving forward to be able to differentiate between them. They are not the same as major barre chords. They are a triad with some interval of a seventh added. This will make more sense after you look up the different seventh chords.
I think that particular video is confusing because he starts talking about the G major scale in the first half of the video and then moves to a G Phrygian mode. So separate those parts of the video to help understand.
The previous video you saw must be referring to G minor for C Phrygian. In the G Natural Minor scale (A# Major [Ionian]), the C plays the Phrygian role in that scale.
Don't get TOO caught up in the details of the theory right off the bat. Get an understanding of the ideas and then try to apply that to the instrument. Once you're able to practice it, the theory will begin making more sense. Playing with the concepts will put you closer to that understanding and allow you to more easily digest the theory.
The ultimate point of the "How to Apply Modes to Chords" video is to show you how to highlight the proper parts of the scale based off of the chords being played. Pull up some scale charts for a reference. In his example, he had a Gmin chord and an Abmaj chord right? Check your G minor scale. Is the Ab note in the Gminor Aeolian scale? Nope. So while it may be serviceable, it's not the best option for us. Keep going. G Dorian is a minor scale, too. What about that one? Ugh... nope. the Ab note isn't in that scale either. The only other minor mode we have is G Phrygian. AH! We have an Ab note in this scale! And look! it's a major tone because it's the 4th note of that Eb major scale Tyler was talking about. So this fits perfectly with our chord progression. It's going to be the most applicable scale to use for the song based on the chords.
Again. Don't overwhelm yourself with the "on-paper theory" at first. Go over the information, look at examples and then try to copy that. Don't just focus on playing though. Focus on what you're doing and try to make sense of why it works. Hopefully you have a looped. Record a simple Gminor to Ab Major chord progression and then play all three minor scales listed above. You'll notice a difference between them and the Phrygian will stick out as sounding the best. Once you do this, you can start to break down what you're doing. Look at the notes your fingers are pressing down when playing those G minor and Ab major chords. Notice that all of these notes are being played in your Phrygian scale, but not the aeolian and dorian scales. THIS is why that chord sounds good. Start to piece together these ideas and you'll take the next step!