Is this scrubber good for bathroom and bathtub cleaning?

Yes. It is aimed at bathroom, bathtub and kitchen surfaces, and the rotating brush reduces the hand scrubbing those jobs usually need. The extendable handle helps with tub walls and higher tiles.

How long does the battery last on a charge?

The listing gives a 2000mAh battery and a 5V USB-C input but no runtime figure. Expect shorter runs on the higher speed setting and longer ones on the lower setting.

What is the difference between the 3-head, 5-head and 7-head kits?

They differ in how many brush heads come in the box. More heads means more brush types and spares for different surfaces; the scrubber body itself is the same.

Does the handle extend?

Yes. The handle is extendable so you can adjust reach for higher or lower cleaning areas.

Can it replace a corded scrubber for heavy grout?

Probably not. The ABS housing and 5V charging suggest a light-duty tool for everyday surfaces. Stubborn grout and set-in stains are better handled by a more powerful corded model.