Google translation: "Storage location for XMP filesI'm new to Lightroom and have enabled simultaneous storage of XMP data. Although these were obviously created during synchronization, I cannot find an XMP file on my computer. Where is this data stored by default."
Do you mean that you have checked the option Catalog Settings > Metadata > Automatically Write Changes Into XMP? If that option is checked, LR will only write the XMP metadata back to disk when the photo's metadata changes (e.g. you make edits in Develop or change a field in the Metadata panel).
What types of files -- raw, DNG, JPEG, TIFF?
For raw files, LR places the .xmp files in the same folder as the raw photo.
For non-raws (DNG, JPEG, TIFF, PSD, PSB, PNG), LR doesn't create .xmp files -- rather, it stores the changed metadata back in the photo itself.
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