“Build files exist” means that yay already downloaded and the packages files. I think this might be the case if for example you already tried to install the package but aborted before installing it.

“Clean build” means that yay will re-download and re-build the package’s files.

The numbers are examples for choosing to which package apply which option which package to cleanbuild. ( “1 2 3” selects packages 1 2 and 3. same goes for 1-3, but in this case you are specifying a range. ^4 means all the listed packages but no. 4). this is an example output, and in you case you can ignore it given that you only have 1 package.

I think pressing enter for the default is okay, otherwise type “A” and then enter

Well my account is not that old, but i did my part.