Is Jet-A used in turbojets or is it just used in turbofans, turboprops, and the SMA SR305-230 for Cessna?
Jet-A is mostly for turbine engines in general. Turbine engines can be fueled with nearly anything that you can spray over an open flame and will burn.
It is not a substitute for avgas, for the same reasons that diesel is not a substitute for petrol. You can fuel a diesel engine with Jet-A under some circumstances.
Jet-A/A1 is used by pretty much all civilian turbine aircraft. The US military use JP-8 which is similar but with extra additives in pretty much all their aircraft.
The fuels can also be used in suitably designed diesel engines. AIUI the US military runs pretty much all it's vehicles off JP-8 and some airports use Jet-A/A1 for their ground vehicles. In both cases the motivation is simplified fuel handling.
Due to cost/availability/environmental concerns with avgas there has been a push to develop aviation diesel engines that run on jet A/A1 (and in some cases can also run on automotive Diesel). However the GA market is relatively small, certification costs are high and making a Diesel engine light and reliable enough for aircraft use is not easy. So-far only a small proportion of piston aircraft have Diesel engine options available.