"What is to become of them?"

What is to become of them? = What are they to become?

What has become of them? = What have they become?

What will become of them? = What will they become?

I was wondering if the three equivalences are correct.

What kind of usage is that in general, not just for "become"?

Solution 1:

It usually means that you're asking what will happen to them / how they will end up.

I've never seen it used to ask literally what they will become.

Solution 2:

The phrase become of has its own definition. I quote the OED:

become of (after ‘what’) was used formerly in sense of ‘come out of, result from,’ but has also taken the place of ‘where is it become,’ etc., in 1b, in reference to the later locality, position, or fate of a person or thing.

In modern usage, it basically means "What will happen to them?"