Will a 36 inch induction and telescoping downdraft fit in a standard cabinet or do they require a custom cabinet?
Should have said standard depth cabinet.
Yes, No, Maybe...It depends...on several things.
First carefully read the manufacturer's installation instructions while keeping in mind your countertop material and your specific cabinet.
Was the cabinet built using 1/2" plywood for example, or 3/4" material? The interior space changes accordingly. ALSO, what type of counter top is in play? If it is granite check with your fabricator. Big holes (like for cooktops) cut into granite will usually be set back from the edge further than for some materials. That could also push the downdraft further back in the cabinet.
Do you have an option to go any deeper?