How do schools determine their curricula?
(This applies to US universities; not sure if the rest of the world is similar.)

Textbook decisions are, by default, made by the individual professor teaching the course, based on what they feel is most suitable for the course.  They would of course consider how the course fits into the degree program(s) that require it, but there usually isn't a concerted effort to have consistent textbooks across all courses in a degree program.

For "standard" courses that are taught regularly by many different professors, the department may choose to adopt a standard textbook.  This decision would typically be made at the department level, led by the professors who teach the course most frequently, with input from others in the department, other departments whose students often take the course, etc.  For courses in which a particular piece of software is central, the choice of software might be similarly standardized.

Higher administration usually has little or no direct role in such decisions.

Supplemental materials (additional software tools, videos, extra reading, etc) are usually selected by individual professors, and are not usually "standardized" like the main textbook can be.

Bookstores will by default stock all books and other materials that are listed as "required" or "optional" for the course.  Everything beyond that is at the discretion of the bookstore manager, based on whether they feel it makes business sense.