Large companies such as Google usually provide their own internet service.
Because Google Has Google Fiber Internet that is offered to people who are in the area that can access the fiber internet from Google.
Then the Google Fiber connects to other ISP companies to allow the internet to flow through otherwise they would not get internet themselves.
Their data centers are also connected to many different ISP companies through fiber optic connections to other data centers.
When you connect to Youtube or Google your ISP goes through many other data centers and ISPS within a second to get to Google or Youtube.
Then that data is sent back through Youtube, Google and many other ISP companies to send the data back too you.
So Google has connections to many other ISP companies for the internet to flow through at their data centers and they have their own ISP usually at their Google Office and Youtube headquarters.