Why does Overleaf not compile the current file?

Question

In some of my projects, Overleaf would not compile the current file I had open (but rather the original file that was open), while in other projects it would always compile the current file. Any idea why this might occur?

Answer 1

Turns out that the issue was that I did not have the \documentclass command written at the top of the file (I had instead written this in a separate preamble.tex file and done \input{preamble}). If there is no \documentclass command in the file, then Overleaf will not compile it when you switch to that file!