As we know,it takes at least 10mins to transfer bitcoin to other accounts.therefore I guess they actually don't transfer any bitcoins except when their users try sending to others they want.
So do you think what I guess is right?
Transfer of a transaction is immedeate. You can send from one address to the next, and it appears within seconds. However: it is not confirmed (in a block). It takes in average 10 mins, that a new, valid block is created. The block is made of many transactions, that were validated before by the node. There is a mathematical puzzle to be solved, which is adjusted from time to time, so that the "block time" is always equivalent to 10min in average. You can send your tx anytime you want, it just might take some minutes, until it appears in a valid block. I ercommend reading some more, maybe the online available book of Andreas ("Mastering Bitcoin"). It explains pretty well the details.