but there are criteria for enrolling outside of the fixed periods.
I am not sure if just getting a new job is one of them.
but if you had prior coverage while unemployed (medicaid or similar) then switch to a job that removes medicaid eligibility, I believe that is a valid reason to be allowed to enroll.
For me it was gap coverage between jobs I enrolled for. this qualifies.

loss of insurance coverage if you are fired, employer stopped covering you, etc. There were a couple of other reasons but I dont remember them at the moment.

as the specific details concerning the ACA are very state-specific.

Go to https://www.healthcare.gov/

Give it your zip-code and it will tell you if you can qualify to use the Federal Marketplace or it will link you to the state marketplace. If you are eligible to use the Federal site you can continue on and find out what events would qualify you for an special enrollment.

I bought through the market place until I qualified for medicare.

They will walk you through everything if you're not comfortable with the website.

I am in Oklahoma, and it look like I would qualify for a mid-year enrollment.