The main source file is Inter.glyphs, but you can easily generate
UFOs and work with those instead, if you're using a font editor that does
not support .glyphs files.
For more details on how to work with these source files and how to build font files from them, see CONTRIBUTING.md.
make designspace
This will generate all masters as UFO files in this (src) directory as well
as accompanying designspace files.
You can now forget about the .glyphs file and simply work with the UFO and
designspace files, and keep building using make.
Note that UFO and designspace files are automatically generated as part of
the build process, so in practice you can simply run make and those files
will be generated for you.