This depends a bunch on what your goals are. There are no specially named files, so looking for a particular filename is not particularly useful. It is sometimes informative to find the file containing the main, but not always.

My job at NCC Group involves a lot of code reviews, so frequently the files that are of interest to me are the ones that contain the most defects. I typically identify these by compiling with compiler warnings turned up and warning suppression turned down. I'll frequently also make use of static and dynamic analysis, including the GCC and Clang sanitizers.

It all depends on how organized previous workers were, and what your goal is for a modification of the source text. Often, headers (dot-h files) document the data structures and interfaces.

I start with generating tags.

  exctags --exclude=TAGS --exclude=TAGS.NEW --append -R -f TAGS.NEW --sort=yes  && mv TAGS.NEW TAGS

My editor (vim) has native support for quickly jumping from a use to definition via this TAGS index. History is preserved (i.e., there is a "back" button), so you can quickly dive through 5 layers of API and back out to understand where a value went. It is quite useful for starting with what you know and following it to the surprising behavior, without executing the code.

cscope can help

Is there a vim-style cscope interface for emacs? I hate that xcscope brings up its own persistent buffers (replacing other buffers that I had deliberately placed on the screen). Vim, conveniently, just pops up the cscope interface when I need to enter some input, and then hides it away. Also I don't think xcscope works with evil's tag stack whereas in vim, I believe, you can just return to where you were with ^T, whether using ctags or cscope.

Yes, I have found it helpful. One nice feature is that it uses a character-terminal interface, not a platform-specific GUI.