Actual metadata sidecar files are generally in the XMP format.
The jpeg has dimensions 4592x3448 and a light amount of exif, and comes with a paired rw2 file which has dimensions 160x128 and only contains an embedded jpeg thumbnail of that smaller dimension and a rich amount of exif. The jpeg is 6.31MB on disk, whereas the rw2 is only 1.76MB on disk (clearly not an uncompressed TIFF + metadata). The photos were taken by a Lumix GX1 between 2012 and 2017. Looking through the hundreds of directories of images, that pattern is clear that the jpeg is 4x the paired rw2 on disk.
I can no longer use the official Lumix codec as it is no longer supported ( https://av.jpn.support.panasonic.com/support/global/cs/dsc/d... only installs on Windows 7 and below), and DX Photolab doesn't know what to do with it. FastStone Image Viewer reports the smaller file size, and reveals some of the exif (interestingly the values in the rw2 are more accurate, the values in the jpeg are rounded). The Adobe DNG converter produces a 160x128 jpeg from the rw2.
The things I read only agree with you, the files on disk do not.
An example of that is Panasonic Lumix cameras would take a JPEG+raw. The raw is a .rw2 file that is pure metadata, but carries enough information about the camera and lens ability and parameters used to allow effective transformation of the JPEG as if you truly had the raw.
In those cases you had two files with exif data: the JPEG with basic exif and then the sidecar file with a really rich exif.
Raw drivers for Windows by Panasonic Lumix were really just sidecar parsers.