Pressure cookers ARE NOT “plug-ins”. They are pressurized vessels made out of heavy gauge aluminum that are operated with an external heat source. Commonly used for canning they have a lid that locks into place with a substantial gasket to avoid leakage and a pressure relief device.
There is a BIG difference between a pressure cooker and your typical kitchen appliances like rice cookers and crock pots.
They would be used in bomb making to allow the explosive pressure to build up before blowing open the vessel.
In my teenage years a recall making sparkler firecracker. Most either fizzled or the casing was so tight the explosion sputtered. It would seem to be that the person making the bomb had good training to know the appropriate ratios. Either the person was trained abroad or had to have live test in the US. Either way, this leaves a trail.