The text isn't supposed to show up. It's alternative text for viewers that can't see the image (or otherwise need it described).
In web browsers, you often can long press the see alt or title text, though. As an aside, title text is kinda similar to alt text, but shows up on hover in desktop (on mobile, it's also long press). It's not meant for accessibility and not usually friendly for it. It's usually for supplementary information and not to describe the image. Eg, an acronym might have title text that defines what the acronym stands for.
You shouldn't do that. That's not gonna help screen readers and in fact will just waste their time trying to read off a URL because it thinks it's describing the image. The alt text is intended to be human readable.