Why 72ppp for web files?

when you paste from a website this forum includes the style info by default because the engine its built on is designed for web media and not forums i.e, Adobe cut corners and this is one of the side effects

https://www.photoshopessentials.com/essentials/the-72-ppi-web-resolution-myth/

When you use Export or Save for Web, and then upload the file to the web, it has no ppi value, it's been stripped out.

The exception is if you choose All metadata in the Save for web dialog.

On a side note, how do you manage to create posts that have to be scrolled horizontally?

If the resolution of a file is only taken into account when printing, because the resolution parameter appears when creating a web file, if this parameter only takes you into account when printing. And why is it set to 72dpi ?, since changing the resolution without the option to resample does not change the file size. Is the belief that web files have to be 72dpi false? That does not reduce the file size. The file size for a monitor will always depend on the number of vertical and horizontal pixels, not the resolution. So why does the resolution appear when creating a web file and why 72?

Your assumptions are correct.

Ppi is optional metadata, and is used by printer drivers to calculate printed size.

Pixel dimensions divided by ppi = printed dimensions in inches.

On screen, images display according to their pixel dimensions, regardless of the ppi value.

When you use Export or Save for Web, any ppi value is stripped out, but if you open the exported file in Photoshop, it has to assign a ppi number, and 72 is the arbitrary number used.