Xcode - But... Where are our archives?

• Open the Organizer window in Xcode (menu Window > Organizer)
• Click on the Archives icon in the top middle
• Select the desired archive by app name and date
• Click Show in Finder in the context menu

Xcode menu bar > Window > Organizer > Products > Archives. It is also called the Archive Organizer. The archives are stored in /Users/username/Library/Developer/Xcode/Archives/$DATE/Project Name Datetime.xcarchive

Archive location

By default location is on the same level as Derived Data folder

<some_path>/Library/Developer/Xcode/Archives
//e.g.
/Users/alex/Library/Developer/Xcode/Archives/2021-05-08/ServiceLocator 08.05.2021, 15.35.xcarchive

To change location

xcodebuild archive -archivePath "<some_path>/<some_name>.xcarchive" <...>
//or
Xcode -> Preferences... -> Locations -> Locations -> Archives 

To show Organizer

Window -> Organizer -> Archives

Just to add to this since I'm unable to edit @Codo's answer, but for my version of Xcode (12.3) there was no Archive icon in the top middle. Just a slight difference. Instead, going to Window > Organizer opened a window displaying all my Archives for the current project. Right clicking each shows a menu to delete Archive entries or you can Show in Finder.

Show in Finder shows mine are currently located at /Users/username/Library/Developer/Xcode/Archives and within that directory there's a folder for each date which contains an Archive.