On Visual Studio version 16.6.1 (on Windows), when building a Xamarin.iOS app with a Mac as a build server, the "Show IPA File on Build Server" option is grayed out.
So where can I find the IPA on the Mac?
(I already searched in the Mac's HD -> Library -> Caches , but there is no "Xamarin" folder there.)
You are using the wrong Library folder.
You need to go to ~/Library/Caches/Xamarin/. Then the builds will be in mtbs/builds folder.
Where ~ is your user's base directory.
NB: Keep in mind that the user's Library folder is hidden, so you will need to either show the hidden files (cmd + shift + .), or to navigate through the terminal or with Finder's Go to folder... options (cmd + shift + G or through the menu Go --> Go To Folder...).