For unverified users on Binance.com who have multiple altcoins and want to withdraw everything to another wallet, how does it work? Do you send the coins to the wallet one by one?
Yep, you have to send various coins one by one, and for each you have to pay withdrawal fee so I suggest that your convert all of them to BTC in order to lower the fees. Unless of course you plan to hold them for longer period of time so you really want to keep those altcoins.
Do they convert the coins to bitcoin upon withdrawal so the receiving wallet gets the coins in bitcoin?
No. You have to exchange those coins for BTC by yourself, and then withdraw it to your BTC wallet.