how to install 15 on older MacBook pro

Here is ➡️ How to upgrade to macOS Monterey - Apple Support

You’ll need somewhere between 26 GB and 44 GB free storage, and quite possibly more, to install macOS 12 Monterey.

iOS 15 is for iPads and iPhones. Monterey is mac OS 12. And here are the requirements for it

https://techtoschool.com/blogs/news/macos-monterey-system-requirements