I prefer to split up bigger projects into multiple files, where I use the following "magic comment" to point to the root file:
% !TEX root = ../my-root.tex
Sometimes, I have to search globally for a certain string, hence, in multiple files within the current project. Atom has this neat "Find in Project" feature you can use with
cmd + shift + f respectively
ctrl + shift + f.
Does TeXShop has something comparable? The standard search dialog only includes the current file.
EDIT: I just found out that this has been on the mailing list at least since 2011. But as far as I can see there is no official issue tracker, where one can check the status of feature requests like this.