So when you leave your car in a garage in the city, based on the contract you have with the owner of the garage (usually via T&C), you don't own your car anymore?

Ownership is not the same as possession. You don't possess those coins, but you may legally own them. The contract may specify that the coins cannot be rehypothecated for example, so it's pretty much the same as your parked car. In both cases, your property may not be returned, but in both cases you may be OK with leaving a small part of your net worth with someone else for convenience and efficiency.