How do I learn the reasons for the moves in openings

I am not selling books, and I have no share in them.

Indeed. Merry Christmas to all! 🎄 My best advice to you~Play play play these openings without pretense, expectations and from a curious perspective. Use previous knowledge, that you had garnered to this point (as far as opening theory is concerned) and apply it in analysis whether you had won or lost using these openings. Run through the lines in MCO to it's evaluation point and play out the game from there. See if the GMs, Masters, who wrote the openings gave sound evals. Compare it to your own perspective of the positions. That is the essence of delving into the variations; of understanding... it's only by your own volition, through study, practice & gaining familiarity through experience. Your own journey. Not only from buying books, forums. Application with a purpose and not just by blindly abiding by what others say it is. Best wishes. Peace✌🏽

Reti's Masters of the Chess Board taught me much about openings.  Just knowing what you are striving for after playing d4 or e4 will get you quite a long ways.

"... Reti … goes into considerable detail as he explains each of the popular lines of the day, … True, some of his comments about specific openings are positively ancient. … Masters … was published … in 1930, …" - GM Andy Soltis (2012) who nevertheless thought that the book was a "classic"