My assumption has been that planes mostly don't have air conditioning. I was stuck on the ground due to technical issues once and so they brought in an air cond

My assumption has been that planes mostly don't have air conditioning. I was stuck on the ground due to technical issues once and so they brought in an air conditioning truck to keep things comfortable (this was in South Texas). I thought they brought in bleed air from somewhere in the engine which would be pretty cool at altitude. I'm sure someone who actually knows about airplanes could chime in and correct me.