The way I would describe it is:
You use the post dialog and attach files using the button just below and to the left which has the paperclip icon.
The attached file is uploaded to a directory on the forum server. The attached files, presumably pictures but could be other file types, is associated in the resulting data with your user ID number.
The other way to allow an image to display in a post is using a different web hosted location to store the files, and then use "bulletin board" codes, or bbcode to insert a command that allows the forum software to display the remotely hosted image/file.
If by 'normal you mean pasting a copied image into a post from your clipboard on your device; the image is still hosted by forum software but likely does not show up in the "attachments" dialog but is rather stored in a different structure in the forum software.
I am not very versed in the system workins' of XenForo, the forum software, so the above is an educated sketch of what I think is going on.
XenForo is a cloud based package as opposed to the "good old days" of having to install, debug, upgrade and keep patched the forum software, work that would have to be done by the owner/admin or their delegates.
Now it is all done by the Xenforo folks under the managerial direction of the customer, eg the owner.
The reason I mention that is to illustrate that the owner/admin is abstracted from the database and file structures, as well as deep dive diagnostics, and therefore not privy to the goings on within the system.
I also don't use or have any experience with the mobile app interface.
Or, it could all be a conspiracy. I dunno.