Who insures when you have switched employers and you have to wait 90 day for the probationary period?
During a 90 day or less waiting period for employer health insurance you should keep your current insurance, get COBRA, or get a Marketplace plan.
You should not drop your insurance in between jobs, if you do risk going without coverage you can use COBRA or health insurance marketplace coverage in the meantime. You can use marketplace coverage without cost assistance even if it is just for a short while. Simply enroll in a plan at least 30 days before your old plan ends and then cancel the plan so it ends when the new one begins (at the end of your waiting period.)
If you didn't have a plan and are waiting for coverage and it is during open enrollment you can enroll in a Marketplace plan. If you didn't have a plan and are outside of open enrollment you may not have coverage options outside of short term health insurance until your employer coverage starts.
Learn more about how special enrollment works so you can get a plan in between jobs.