C172 pilot reported an NMAC while on approach to a non-towered airport requiring evasive action. The conflicting aircraft was a Lear jet landing at a nearby airport. Reportedly; the Learjet jet was lower than published for their glideslope.

Narrative

While in-route from the NW Training area with students. I was @10 miles out from my destination ZZZ; Approach canceled flight following; and advised me to switch to ZZZ UNICOM. I was approaching the airport from the west/northwest at @1;500 MSL. My aim point was to overfly the airport at midfield and enter a left downwind for Runway XX.The UNICOM chatter and air traffic at ZZZ indicated they were using Runway YY. I announced my intent on of where I was at in relation to the airport at my attitude as I began my approach. Note: My current altimeter was set from the data that Approach provided me earlier. Someone on UNICOM announced they were on left downwind to Runway YY. I announced and entered the left downwind at midfield descending to 1;400 MSL. I continued my downwind at 1;400 MSL at 90 knots looking for the other aircraft to base then final before I would turn left base for spacing. I radioed on UNICOM for the other aircraft to give a position report. A pilot behind me in another aircraft radioed and said Aircraft X; you are number one on downwind and he will be number two following me. At the same moment my TCAS indicated there was traffic right on top or right below me! I tried to fly away from it; and looking all around; I saw nothing! Maintaining @1;400 MSL; I announced on UNICOM that Aircraft X turning left base Runway YY. At that moment a Corporate Jet descending from my upper left from my blind side to in-front of me; missing me by @50 - 75 feet! This was very upsetting! This Jet was in-route to Runway XXL at ZZZ1 [Airport]. The published attitude to approach at ZZZ from where I was is a glideslope 2;200 MSL. He was well below it. Why didn't ATC notify him that he was low and why didn't they (ATC Controller) inform the Lear Jet pilot that he was on a collision course with general aviation traffic at ZZZ. I've filed a verbal report of this Occurrence with ATC at ZZZ1 the same day.