After reading the FAQ's of genesis mining, it is clear that it is a kind of pool mining company, however, you do not buy the mining rigs for yourself but just pay them for the hash rate you wish to by. Now the amount of bitcoins you receive is decided by the ratio of your hash power to the entire hash power of genesis.
As an example, say your hash input is 1 hash/sec and the total hash power is 100 hashes/sec, and genesis mined a single block that day. Todays block mining rewards are 12.5 BTC. So you will be paid 12.5/100 BTCs. However the FAQ's also mentioned that they cut certain operational charges. So you will get a little less than the above figure.