It’s really difficult, and I think you have to absolutely keep eyes on them until they get old enough to understand danger.
Suggestions: multiple barriers to danger
Make toddler bedroom a safe containment area:
Small zip up tent with mesh door, able to be locked from outside, to contain toddler when an adult can’t keep eyes directly on them.
Nothing in bedroom they can climb on. Consider a mattress on floor.
Lock on bedroom closets and door. They need to be locked in at night: bedroom AND all exterior doors. Set exterior alarms.
Locks on bedroom windows. Tape on window glass to minimize damage to toddler if they try to break out. Window bars if possible. No blinds, curtains, CORDS, etc. No framed art, mirrors, etc.
Absolutely hide all keys. Keep car in locked garage. Create as many barriers as possible between them and cars. Do not leave them outside alone, even in fenced yard.
My mother said she began to understand why some children might be locked in dog runs for their own safety. I don’t think that is a terrible idea. Some are large enough for children to walk around comfortably, but I shudder to think how social services would react. If I had it to do over again, I might crate mine for safety when I couldn’t keep my eyes on them… No joke. Lots of trips to emergency room, but they finally out grew this stage.
PS. Be sure they understand they are physically incapable of flight, even if wearing the correct super hero cape… Perhaps allow them an attempt from a low chair so they believe you.