Bring in the files

a zip of the project
Choose or drop project.zip
the .tex and .bib inside are listed for you to pick
or one file at a time
or paste contents after adding it
Files never leave the page except in the one model call you trigger in stage ii.

Mark files as .bib (bibliographies) and .tex (sources to scan). Several .bib files can be merged into one below — later stages work on the merged bibliography. A bibliography on its own is enough to verify; repairing needs at least one source too.

Either order works, and either half on its own is a complete job.