Reusable code is usually pulled out into a library and reused that way, rather than copied and pasted into a new project. You might copy and paste some boilerplate to new projects but it wouldn't be anywhere near 25% of the code.

I'm not sure why someone downvoted you (it wasn't me!) because your comment did seem like a genuine question.

Pretty often, but then you can just refactor the code so you can use it for more situations

What LLMs are good at are the opposite - when the thing you want to do is almost exactly the same, but nearly all the details need to be changed

Say you want a page to edit account details, and another page to edit community details. And the API paths to do this will be even more similar - but because they're different things, you'd have to get fancy with the design to make code that works for both... It's possible, but there will be trade-offs

LLMs are great at it though... Pass in the account page, give it the object definition for the community details, and it'll spit it out for you