PhotoLab automatically synchronises metadata from .xmp files and the PhotoLab database. Newer parts replace older parts. If e.g. the database has newer metadata, all metadata from the database replaces what’s in the file, be it .xmp for RAW or image file for non-RAW files. Not every metadata field has a timestamp, only the complete set has and that is why the sync is either “read all” or “write all”.
To be in control of what metadata gets read or written, it’s necessary to disable autosync in DPL’s preferences/settings and read or write manually with the respective menu entries.
Nevertheless, metadata should be written to output files, unless the boxes are unchecked in the output dialog. Running a short test with DPL 7.0.2 on macOS 14.1 shows that DPL exports IPTC and GPS to JPEG files indeed. I also found that DPL writes IPTC in XMP, while Lightroom 13.0.1 writes IPTC in IPTC, which is the old (legacy) way to write IPTC tags. IPTC Standards currently promote writing IPTC tags in XMP.