Most manufacturers do in broad terms disclose how the boot sequence works. Here is an example of the information disclosed about a Cypress microcontroller [1]. In some applications the startup time can be critical so this needs to be known. The code that runs before the engineers code doesn't usually have any effect and it puts the microcontroller into a known documented state so you don't really need to think about it.

[1] https://community.cypress.com/community/psoc-6/blog/2017/05/...