how does OP.GG get their stats?

Everyone i know who plays league, (including myself) uses this site. the stats are extremely useful and interesting to look at. Today while looking at some champ stats, i was wondering, how do they get these stats? At first i figured riot must disclose some information and op.gg just uses the stats they were given from riot. But im not 100% sure because they also might make their own statistics based on their own player database. For example, they have record of every live gave and the champions used, and items used in that game, so they enter that information into their own database and calculate the winrates that way..

Anyways, does anyone know how it actually works? and if they calculate the statistics themselves, how do they have access to EVERYONES live games, can i find this information myself without using op.gg? just curious