Every two stroke engine must have a way to pump the air into the combustion chamber against the positive pressure of the remaining exhaust in there. The more complex 2-stroke engines use a mechanical blower sometimes augmented by a turbocharger. Simple 2-strokers use the piston underside as the pump. Here the crankcase for each cylinder is sealed separate from the crankcase spaces under the other cylinders (multi cylinder outboards a an example, as well as 2 cylinder motorcycles).
The simple gasoline 2-stroke engines mix the gas with the air, pull it into the crankcase when the piston goes up, then push the air & fuel charge into the combustion chamber when the piston goes down. There is no heat source in the crankcase hot enough to ignite the fuel & oil mixture.
Very large two stroke diesels (getting up to almost 9,000 hp per cylinder) use multiple turbochargers, with electric blowers for slow speed. The piston underside is still used as an additional air pump getting the air from the turbocharger & after cooler.