Some publishers' system only compiles submitted .tex files in a single pdflatex or latex run, without running bibtex; hence the instruction to authors who used BibTeX during manuscript preparation to paste the contents of the .bbl file input into the .tex for submission. Here are the two ways you can do this on Overleaf.

Downloading files from the cache

The .bbl file is stored, along with the other files created during compilation, in a cache. You can download any of these files by clicking the file icon beside Recompile, and scroll down and click Other logs and files.

Back in your .tex file on Overleaf, comment out the line \bibliography. And in its place, paste the contents of the .bbl file downloaded from the cache.

Downloading the source including the bbl file

It's also possible to download the source code with the .bbl included. To do this, you can click Submit, then select arXiv, and click the option to download.

Then, you can unzip the folder and copy the contents of the .bbl file into the main file in place of the \bibliography command, if needed.

The project is now ready to be submitted to the journal, complying with their requirement that the contents of the .bbl file are pasted in the .tex manuscript as one single file.