Learn Hardcore Malay: Sebahagian bangunan ini ditutup.

Sebahagian means part, a portion, or some of. In this sentence, it tells you that the statement applies to only part of the whole, not all of it.

So sebahagian bangunan ini can mean either:

part of this building, or
some of these buildings

The exact meaning depends on context.

Why is there no word for of in sebahagian bangunan ini?

Malay often does not need a separate word for of when linking nouns.

So:

sebahagian bangunan ini = part of this building / some of these buildings

If you want to make the relationship extra clear, Malay can also use daripada:

Sebahagian daripada bangunan ini ditutup.

That is also correct, and many learners find it easier to understand because it more directly matches English part of.

Does bangunan ini mean this building or these buildings?