since google is tier 1 network. does anyone knows if they pay for bandwidth for serving youtube?

They aren't a tier 1 but they also aren't paying for most of their bandwdith. They peered very liberally in the past so have zero settlement peering to most networks and if you're a big enough eyeball network you can request a caching server from them where you pay for rack space, power and network and google just pays for the server hardware. [0]

[0]: https://support.google.com/interconnect/answer/9058809