No. Redmi 6A has 2 dedicated sim card slots and a dedicated MicroSD card slot. So, you can use 2 sim cards and a memory card at the same time.
Yes, you can play PUBG on Redmi 6A.
No. Redmi 6A does not have Dolby Digital sound support.
Yes, Redmi 6A does have the fingerprint scanner.
The Redmi 6A is a fairly good update to the Redmi 5A, which itself was barely any different compared to the Redmi 4A. You now get a tall screen, better processor, and improved polish overall.