Can it charge a laptop?

Yes, up to 45W over USB-C Power Delivery. That is fine for thin 13-inch laptops and Chromebooks, but larger or gaming laptops will charge slowly or only hold their level.

How many devices can it charge at once?

Three: the built-in USB-C cable, the spare USB-C port and the USB-A port. Total output is shared, so each device gets less when several are connected.

Is the retractable cable replaceable?

No. The cable is built into the charger and cannot be swapped out, so a damaged cable means replacing the whole unit.

Does it come with a separate cable?

No. You get the charger with its built-in USB-C cable; the spare USB-C and USB-A ports need your own cables.

How does it compare with the Anker Nano 45W?

Both are compact 45W GaN chargers. The UGREEN adds a built-in retractable cable and a USB-A port; the Anker has no built-in cable but uses standard ports you can replace freely.

Will it fast-charge an iPhone or Samsung phone?

Yes. It supports USB-C Power Delivery, which fast-charges recent iPhones and Galaxy phones, though you need a USB-C to Lightning cable for older iPhones.