At what altitude? At low altitude, it is theoretically possible (although some passenger aircrafts have a lock).

But even then it would still require significant strength, as there would likely be some air differential due to the aircraft's speed/air pressure against the door.

At higher altitudes it is essentially impossible for a human. You'd have to blow apart the door or equivalent.

The biggest thing is the pressure differential. And that varies based on speed/altitude/etc. At normal cruise in a jet aircraft, it just isn't happening.

I do not get it, sorry. The door opens out, and the pressure is higher inside. So how does the pressure difference prevent opening that door?

It isn't intuitive how aircraft doors work, but the door doesn't open "out" in the typical sense. It actually sits against the aircraft's outer hull, the larger the differential the higher pressure exist between the door and the skin of the aircraft.

When the door is opened, it actually slides in then out sideways rather than swings open.

See: https://en.wikipedia.org/wiki/Plug_door

Most doors on aircraft open inward, so think of it like a drain plug where the pressure holds it in place.

Edit: To clarify, they may swing outward, but they open inward first.

nope - atleast not on your typical commercial passenger aircraft after certain height is achieved. The air pressure difference between inside of aircraft and outside makes it impossible.

No