I don't know about the OSI's open source definition [1] in general, but specific licenses might consider makefiles and build scripts to be part of the source code. (For what it's worth, the free software definition from the FSF does consider makefiles and build scripts to be part of the source code [2].)

[1] https://opensource.org/osd/

[2] https://www.gnu.org/philosophy/free-sw.html

No, it’s not the same. Yes, you can technically modify binaries, but it’s not at all the preferred way to modify the program.