Tips for integrating this kind of harmony into my compositions

Hello good people of Pianogroove!

I’d like to kindly ask for your expertise to help me understand the harmony used in bars 87-88-89 (15.45) of the following piece (definitely worth checking out if you don’t already know it!).

There’s also something similar earlier on 12.50 (bars 35-36-37).

I’d really love to get a clear grasp of this harmonic pattern so I can start integrating it into my own compositions and arrangements.

By the way, is there a course on the site that specifically covers this kind of arrangement? (I don’t really have the right words to describe the concept!)

Thanks a lot in advance!

Nicolas