Are the sentences correct?

They are grammatically correct, but not very informative.

The question What kind of architecture does Tokyo have? seems to be asking for information about the type of architecture found in Tokyo.

It has mostly modern architecture.

It has mainly concrete architecture.

It has many modern steel frame high-rise buildings.

Thanks.

The use of "it" in this way is a little frowned upon. Somewhat informal and clumsy. Better to write:

What kind of architecture does Tokyo have? Tokyo has superb architecture.

What kind of architecture does Tokyo have? That city has superb architecture.

What kind of architecture does Tokyo have? The Japanese capital has superb architecture.

Thanks, Contrex. Your explanation is superb.