It is all about subscription which is stored in HSS and used by nodes such as PGW and eNodeB during the data flow.
During establishment of default or dedicated bearer, QoS information associated with the bearer is transferred to PGW as well as eNodeB.
eNodeB MAC scheduler takes care of QoS. There are so many parameters are considered at MAC layer while scheduling data and channel condition. Each company has its own scheduler. Scheduler algorithm is not defined in 3GPP specification.