how to put two photos side by side in one picture

iphone has a way built in too. open the Shortcuts app, make one with Select Photos > Combine Images (set it to side by side) > Save to Photo Album. takes a couple of minutes to set up, then it's one tap every time.

On a phone the quickest is Google Photos, select both pictures, tap the + or Create button and pick Collage. Works on iPhone too if you install it. On Windows you can do it in Paint: open the first photo, drag the canvas wider, then paste the second one in and move it over. Fiddly but no extra software.