My iPhone 13 got stolen

Let's start with what "Offline" really means, which could be one, some, or all of the following:

• The battery has run down
• The phone was powered off
• The phone does not have an Internet connection
• The thief wrapped the phone in foil or placed it in a Faraday bag

If any of these conditions exist, you can "send" a command to the phone from Find My, but it will not be received by the phone.

I don't know what you mean by "lock". Your phone is locked by the passcode. No one is going to be able to guess your correct passcode in the very limited number of tries allowed. After that, the phone won't even allow any more passcode attempts.

Perhaps you meant to say that you wanted to put the phone in "Lost" mode. If that is the case, then you will continue to see an "Offline" message until the phone can connect to the Internet.

The same is true if you wanted to "Erase" the phone.

So for now, nothing is going to happen until the phone has a good Internet connection. Hopefully, things are "temporary", but realistically the phone is gone.

If your iPhone or iPad was stolen - Apple Support

Be extremely careful if you receive a message from "Apple" or "Apple Support" or similar indicating that your phone has been found......and you just need to provide a bit more information to the phone can be returned to you. Messages like this do not come from Apple. They are scams trying to trick you into providing your passcode, password, etc.

HOW would the thief or whatever even connect it to the internet?

Connect from a PC or Mac using a cable. But.....iPhone thieves would not be interested in connecting the phone to the Internet because they know that the phone can be located by the owner.

The phone could also possibly connect to the Internet using the intermittent Bluetooth signal that the phone sends out.....even if it has been powered off....and assuming the battery has some charge left....If there is another iPhone in close proximity to pick up the Bluetooth signal that is being transmitted from the iPhone. This is similar to the way that AirTag works.

But, iPhone thieves all know this as well. That is why the favored method with the bad guys is to wrap the phone in foil or place it in a small Farady bag.....so the phone cannot send or receive a signal at all.