Will my iPhone 14 Plus fit into a case for a iPhone 14 Pro Max?

Yes. The phone itself will fit the camera will just look a little funny.

The iPhone 14 plus has a height of 6.33” (160.8 mm), width of 3.07” (78.1 mm), depth of . 31" (7.8 mm)

and the iPhone 14 pro max has a height of 6.33” (160.7 mm), width of 3.05" (77.6 mm), and depth of .31” (7.85 mm)

Hope this helps!

yes. yes, it will. i've seen an iPhone 15 plus in a 15 pro max case too.