I think bikes are 6V, as well as stuff like quads, jet skis etc

not even close. I'm not aware of a single vehicle still in production with a 6 volt battery system.

Many kick-start and pull-start engines do not have a specific voltage but may use an alternator wound with a number of different coils to produce different voltages.

Marine batteries are commonly 6v.

Often used in series for more than 6 volts. At least in the applications i've seen, though i'm a long ways from a sea and lakes may be different.

yes, because the underlying cell voltage is much lower than that. Many 6 volt batteries still exist because they are used in series to make the desired voltage. They are easier to move around and transport when they aren't build as all the cells in one unit.

….As part of a bank of batteries producing a higher voltage. I haven’t seen any 6v equipment manufactured for any engine based equipment larger than a couple of KW for a very, very long time (mid 1960s). Lower voltage means higher amperage means more weight, more cost, more heat, more failures. Objectively, the world would be a significantly better place if we had fully transitioned to 24v or higher (up to 48v anyway) much sooner than later.

Interestingly, micro-miniaturization has reversed the trend of higher voltage = higher efficiency, at least for computing.

Some people in the auto industry want to double the voltage again (for efficiency and to reduce the weight/complexity requirements for all the wiring, especially in EVs) but the 12 volt standard is pretty entrenched.

(Note that I'm talking about the voltage used for everything outside of the internal engine/li-ion electrical systems, which already use higher voltages as needed.)

What’s old is new again. Military land rovers from as far back as the 1950s are all 24v.