To manually save metadata changes to a photo in Lightroom Classic, do one of the following:Select one or more photos in the Grid view of the Library module and choose Metadata > Save Metadata To File(s), or press Ctrl+S (Windows) or Command+S (Mac OS).
If you’re working with a DNG file, choose Metadata > Update DNG Previews & Metadata to save metadata changes to the file and also generate a preview based on the current raw processing settings.
There's a lot of stuff that is not save to xmp. A partial list (I'm sure there are more):
sequence of edits
Lightroom Classic slide shows
Lightroom Classic books
Lightroom Classic web-pages
Which brings up the question: Why do you ask? If you are thinking that you can make backups using xmp, these would only be a partial backup, all the information above (and probably other information) isn't stored in xmp.
The default function of Lightroom is to read and write info to the Catalog File. When you initially import image files to Lightroom Classic it will read info from the file or sidecar file. Thereafter while you are working with Lightroom as far as Lightroom is concerned the info is in the Catalog and it reads and writes to the Catalog file automatically.
There is also an option to have Lightroom write info to the file automatically or manually which serves the purpose of making the metadata and edits done in Lightroom shared with Adobe Camera raw and some other editing programs. However Lightroom has to be manually prompted to read metadata from the file.
You need to be aware prompting Lightroom to read metadata from the file can cause edits and other metadata to be altered in the Lightroom Catalog.
The DNG can also contain all the rendering instructions that you want it to, as long as they can be expressed and saved out as XMP data. A rendered JPEG (size you can control) can be saved too. Ditto AFAIK with all the 'data' you can save in a TIFF or rendered image like ratings, keywords etc.
Author “Color Management for Photographers" & "Photoshop CC Color Management/pluralsight"