First Aid typically require that you have some basic Biotech knowledge and access to some bandages (such as plasters, gauze dressings and/or crêpe rolled bandages) and basic tools (such as safety pins, tweezers, sticky tape and scissors). A First Aid kit does not count as a Medkit.
When talking about First Aid you should probably think about something small and portable like this:
https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSF8mTLbw2yJIBo3oz0AXiTfEQymXcwwRe-DA&usqp=CAUNote that a First Aid healing test can also be performed with improvised tools or without proper tools and supplies at all, just that you get a negative dice pool modifier of 2 dice and can't spend Edge on the test.
A Medkit also count as a First Aid kit, but is far more than just a First Aid kit. In addition to bandages and tools etc (mentioned above) it also comes with various drugs and antidotes, a biomonitor, an expert system and possible also a defibrillator system. To take a Medkit healing test you must have access to a Medkit and you typically first need to hook up the Medkit to the subject (which take a minute) so the talk-active expert system can correctly make a diagnose, administer antidotes and advice on proper procedure (Medkit rating can substitute lack of Biotech skill).
Think
https://youtu.be/4b-63pBYKFk?t=129