What an SVG with a picture inside it actually is
An SVG document is a text file, and the element that holds a picture is `<image>`. Instead of pointing at a file somewhere it points at a `data:` URL - your picture, base64-encoded, written out as text inside the document. That is why the file is self-contained: send it to somebody and the picture travels with it, with no folder, no missing link and no broken thumbnail.
Two attributes decide the geometry, and they do different jobs. The `viewBox` is the picture’s own pixel grid - "this drawing is 4000 by 3000 units" - and `width`/`height` are how large the file declares itself to be. Declaring it at 210 mm while the viewBox says 4000 pixels is not a contradiction: it is the entire point of a vector container, and the ratio between the two is the picture’s own, so nothing is ever stretched.
The file opens in every browser, in Illustrator, Inkscape and Affinity, in Word and PowerPoint, and in the uploaders that want an SVG - because it is a valid SVG document, not an imitation. What it asks of the program opening it is that it understands the format *inside*: JPEG and PNG are universal, while a BMP or an AVIF inside an SVG is not something every editor reads. That is the whole reason this page can re-encode the picture as PNG or JPEG on the way in.