Does each miner in a bitcoin mining pool has a chance to win the block reward?
OR XYZ gathers together hashing power of miners 1 to 10 to try to be the first to solve the puzzle to win 12.5 BTC ?


XYZ assigns work to miners in its pool. The work mines blocks that pay BTC to the pool's distribution account. When a block is mined, the pool distributes from the distribution account.


  Is it possible for a solo miner with 10+ GPU rig to win 12.5 BTC block reward even though mining pool XYZ has much greater combined hashing power? I mean is there randomness also involved in wining block reward in addition to hashing power?


Yes. Everybody who is trying to mine a block has the same chance of succeeding with each hash they do. Mining a block is so hard that somebody somewhere succeeds on average once every ten minutes.