Ok.
Before you get any deeper into worrying about scales or modes, or any theory related anything, remember that there is only one theory rule that holds true in all music, for all time, ever.
IF IT SOUNDS GOOD, IT SOUNDS GOOD.
The scale that’s used for most of this is the E major pentatonic scale. If you’re comfortable with that, great. Don’t worry about anything else theory wise.
Make sure that you:
Know the song backwards and forwards.
If you want to get inside the song and really feel comfortable playing on it, learn and play the recorded bass line.
If you can’t do that yet, then I think that is a good sign to not pressure yourself to understand the inner and more theoretical workings of the song.
It would be like trying to learn the hard-core grammar of a language that you don’t speak - it can be done, but it won’t connect very deeply, and it won’t be very applicable.
So.
Step one - master E major pentatonic.
If the first part of the song makes sense with that scale, great. Learn the bass line verbatim.
When the song moves to other chords and pulls in root notes and chords that are not technically part of E major, just remember the only un-breakable rule of music. If it sounds good, it sounds good.
Modal interchange sounds like Music Theory-ese for “and then they thought it would sound good if they used notes that weren’t in the scale they used most of the time”.
The only other thing I’ll mention here is that when a song is in a key, it only means that, in a general sense, there is a place that feels like home or resolution for a song.
It is a rough, and general guide, and has little to do with the specifics of the tune.
Melissa and Haydn’s symphony no 12 are both in E major. They both move through different keys, different melodies, etc. But they have roots and resolutions in E major.
Music theory is a murky pool if you wade in once in a while.
There’s a ton of information to keep sorted, and when the whole structure isn’t built on a solid foundation, it gets real wobbly real quick.
My strongest recommendation is to learn the song by ear, learn the bass line verbatim, and then see if you can cross reference the bass line to the chords of the song.
Usually that will answer 99% of theory questions.
If you are ever wondering -
“But why would they use a D major or a C major chord in a song in the key of E major” then just remember the golden rule of music:
IF IT SOUNDS GOOD, IT SOUNDS GOOD.
I hope this is helpful!
Holler with questions.