I just, minutes ago, walked my dad through sending several existing photos in a text on his Android phone. He knew how to open the text app and find the person he wanted, so he'd gotten it that far.
1) Tap the camera + photos icon. It's pretty small, and between that and combining two icons for quite different actions into one made it so he'd never have guessed that on his own.
2) Bottom third or so of the screen becomes what I think was a live camera view on the left side, with what I'm guessing was a scrolling gallery on the right, but that was gonna be unusably tiny to scroll through, so I had him hit (IIRC) one of the photos that either had an icon on it or said the word "gallery". That's three separate things (Camera, gallery, quick-select photo view) all blended together such that it took me a second to figure out what I was looking at. He had no chance. Tapped gallery.
3) All context is lost as the gallery view takes over the whole screen.
4) You have a scrollable grid of your photos. It's multi-select, but there's not really any way to know that except to guess that it is, and that the select action will be a single tap (it could just as well be that one tap adds that single photo and takes you back to the message screen, and you have to long-tap or swipe or god knows what to add more)
5) Once any are selected, a small(!) "Add" text(!) appears in the top right(!) corner. It's really easy to miss. It ought to be a full button with some color to make it stand out. I kinda know what I'm doing with computers and it took me a little while to find it. I suppose it's top-right rather than at the bottom (which is better for one-handed usability) because elements near the bottom risk accidental taps on Android's "please ruin what I'm currently doing" persistent button row at the bottom of the screen, which blatant UI mistake they just can't seem to bring themselves to fix even after all these years.
6) If I hadn't been there and he'd somehow figured all that out, he'd have assumed they were sent after that. But no, back on the messaging screen you still have to tap the tiny play arrow with some kind of stuff on it (was it the tiny letters "MMS"? I don't recall for sure) to actually send them, they're just queued to send.
[EDIT: To be clear, though, Apple's interface for the same is barely better]
But this is nothing compared to whatever terrible phone + contacts "app" that phone has (I think it's the default for whatever version of Android he has?). The time he had me fix that for him it took me most of a minute to figure out that it had tabs, because they're just represented by thin text rather than anything that looks like tabs or even buttons, and that the problem was that he'd gotten on to the wrong one. The view he had it on, it looked like someone's half-finished UI mockup. When I got it on the correct one, it still looked like that, but with some content filled in. [edit: oh, and the tab header elements were ~1/3 of the way down the screen, not at the very top or very bottom, making it even less clear what they were]
Google: flat-out incompetent at UI for at least 15 years. I still remember the time like 7 years ago that my grandma couldn't figure out how to add contacts in Gmail and it was because they had two buttons the same color and visual weight on the contact form, both of which with copy that a reasonable person would take to mean "add this contact whose information I've just written in the form", but one of them meant "give me a new contact form (and throw away all the stuff I just filled in)" and the only way to have a decent chance at figuring out which was which was to have some clue about HTML layout hierarchy.