Linux

"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

lets start with why you are getting this output: If you see a screen like this your app is pulled from the AUR. The AUR works different from the other repos. While the normal repos download standardized arch packages in the form of tar-archives the AUR takes a more radical approach: get the app on your system by (almost) any means necessary. So the AUR doesn't contain a package but a text file containing instructions. Where to download the necessary files, where to put them, that sort of stuff. In most cases the files is just the source code and your system will compile it according to the instructions in that text file. Compiling means it will turn the human readable programing in to computer readable stuff. In other words, it will create, or build, a standard arch package right then and there, on your system. That's why the text file is referred to both as "Build Files" or as Pkgbuild.

If you look at your screenshot, you see, that it first downloads your PKGBUILD and then shows the file as present (it uses the plural, because in exceptions there are additional files such as systemd-files that are downloaded as well).

When installing apps from the AUR yay will cache stuff to speed up build times. Performing a clean build means not using this cache. In most cases just pressing enter will work just fine.

And now is the part were I must warn you: Be careful with the AUR:

1. Every app gets root during install, meaning that it can do everything it wants during that time. It's generally recommended that you review every Pkgbuild before installing it. Nobody i know actually does that and the AUR is moderated, so there are people that take actions if someone uploads malware, but that's mostly reactive. To my knowledge there never has been malware in the AUR but it is a possible attack vector.
2. The official wiki states: "The AUR is unsupported, so any packages you install are your responsibility to update, not pacman's. If packages in the official repositories are updated, you will need to rebuild any AUR packages that depend on those libraries." yay will mostly take care of that but there is also [3]
3. Most impotently: The AUR is a project of Arch Linux. You are not using Arch, you are using Manjaro. Manjaro is build on top of Arch, but the tend to hold updates back a bit while they check them for bugs. The people managing the AUR don't care about that. So they might push an update assuming your system to have those held back updates and things may break.

In conclusion. If possible stick with the repos and flatpak. If an app is not available this way you can use the AUR. If you use the AUR you can keep just pressing Enter until the app in question is installed. Just don't be surprised if the app breaks two months later because of something stupid the Manjaro devs did

Cleanbuild means not to use previous existing files, the numbers are exemples of a selection of which packages to cleanbuild. ^4 means all but number 4, the others I think are self explantory

Im not here to explain it because other people already did but imma point out that manjaro is meant to be beginner friendly and the aur isnt so i wouldnt use it in your place.