What is difference between model TH17B1023 & TH17A1020? Does it come assembled? If not how many parts to put together?

The colour of TH17A1020 is black, while TH17B1023 is red. That's the only difference. Neither TH17A1020 nor TH17B1023 goes assembled. And you need to put together some parts including the handle, front wheel, side wheels, and other small parts with the main part.