Look up a picture of one without a shell.
Their entire abdomen is curled to wrap around the inside of a large snail shell, and then they just grab onto it in the same way you could by curling a finger around the inside.
Of course they’re not infinitely strong, plenty of predators have ways of yanking them back out.
In addition to what the other answers have said,
1. They have little back feet/claws that grab onto the inside of the shell to help hold it on.
2. They are strong enough to drag the entire shell with them instead of dragging themselves out of the shell. It’s not like you’re carrying a fridge, more like you’ve got a sleeping bag up to your chest and are army crawling.
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