In my experience, I've found information on Chinese crossbows easier to find, especially technical details, than European crossbows. Needham, Science and Civilisation in China, vol 5 part 6 has about 60 pages on the crossbow; that's a good place to start, but there's plenty more (e.g., discussion of archaeological finds, construction, etc. in Yang Hong, Weapons in Ancient China).

How were they made, and what were their features that made them different from today’s more widely known European design?

The main difference, compared with European crossbows, is that the Chinese crossbow had a long power stroke (i.e., the string was drawn back much further). This meant that:

1. The Chinese crossbow was large - the bow needs to be longer in order to be safely drawn back further.

2. The trigger mechanism is closer to the butt.

3. The same energy can be delivered with a lower draw weight, so there is less need to go to mechanical spanning devices (levers, cranks, windlasses).

The bows could be simple self-bows (one piece of wood), probably composite bows (horn and sinew on wood/bamboo, as usual on regular Chinese bows), and multiple pieces of bamboo or wood bound together like leaf springs.

And was the repeater variant effective or more of a gimmick?

It wasn't as powerful as normal crossbows. The tested example that results are given for in Needham has a draw weight of about 200lb, and a maximum range of 200 yards, and an effective range of 80 yards. The repeating crossbows had a shorter power stroke than the normal Chinese crossbow.

Useful for a high rate of fire, at short ranges (where short isn't uselessly short).

Did they use different designs for mounted combat?

The repeating crossbow was recorded as being suitable for mounted use. But Chinese mounted archers usually used a conventional (i.e., non-crossbow) bow.