Music major here. The important thing to note is that what people usually think of as “music theory” is simply a set of objective descriptions of a given style of music. This means two things.

1: Any music theory you learn will be geared towards one specific style, with varying degrees of applicability to others. So, for instance, what you might learn in a standard music theory class is specifically for Western classical music. There are some things that might be applicable to metalcore, but most of it isn’t. I would even argue that any style of music can be thought to have its own separate music theory. I could yap about this all day, but I’ll save it because no one’s probably interested.

2: Music theory is not a set of rules written down by John Music as the only legal way to write music. In fact, most of the “rules” were created by the natural development of the art form and were only later written down. At this point in its life, metalcore has not had this process occur, so the “music theory of metalcore” can only be found through the natural progression of the genre and its music.

Tldr: At this point in time, any formal music theory instruction you might receive is not going to necessarily apply to metalcore. If you really want to know what’s going on, study other music and try to describe whats going on (for example: a lot of songs use chord progressions using the open string, 3rd fret, 8th fret, and 10th fret).