Step up converters DC or AC?
That link is for a Step-Up transformer. Relying on coupled magnetic flux, a transformer can only convert AC.

A Boost converter is typically a DC-DC step-up converter.

A Boost converter can be used in DC-AC conversion typically called an Inverter; where a DC voltage is Boosted to a higher DC voltage and converted to AC.

A Boost converter is also an important component in a PFC (Power Factor Correction) circuit.