This is the result of the site owner’s struggle to understand algebra (i.e., theory of groups, rings, fields, etc.) with the starting point based on Modern Algebra by professor Akira Hattori.
There is no claim for originality; in fact, it is a hodge-podge of citations from literally hundreds of textbooks and articles.
Caveat: The chapter 12 Algebraic K-theory is only sketchy.