Ignition System Question
I am not eager to answer questions that require watching a youtube presentation, even one that gives the relevant time point. I think some others here feel the same way.

A 12 volt coil will work with a 6-volt battery. It will just give a smaller spark. You can probably find a 6-volt coil for a motorcycle or perhaps a tractor. New coils are even sold for old cars. There may not be any difference between a 6-volt coil and a 12-volt coil. When cars switched to 12-volts, some used the same coil to get a higher spark voltage.

A lot of automotive people often use the old term, "condenser" instead of "capacitor." The value is usually about 0.1 microfarad. If you buy one in an auto supply place, it will probably be identified only by a part number that you will find in a directory that lists car models and condenser part numbers.

There is more to safety than grounding. You should probably be able to find instruction material about that on youtube and elsewhere on the internet.

People have been using ignition coils for projects and experiments for a very long time. I remember reading about projects involving Model T Ford coils in books that were old when I first started doing such things about 60 years ago. Search for ignition coil projects or experiments on the internet. Read several that include text, diagrams and photos as well as more on youtube. You should be able to find lots of additional information as well as safety instructions.