Laws being broken?

Annexation and settlement of a militarily occupied territory is illegal under international law, as a member of the international community Israel claims to follow these laws.

The West Bank is considered occupied territory, not only by essentially every country in the world but by Israel's own supreme court.

How is it apartheid?

Palestinians in large parts of the West Bank live under Israeli governance, and often under direct Israeli military law. Israeli settlers living in the same area are beholden to the much more lenient Israeli civil law.

Two groups of people live under the control of the same state, but under two entirely different legal systems, and are treated by the military and police in very different ways.

That's literally apartheid.