Where Do Companies Like Modash, Clay And All Are Finding Linkedin Datasets?

I recently did few meetings and setups with companies to understand the intelligence they are providing, and one thing was common they all had 100-500M datasets of linkedin user profile and contact info. I am very sure there is a very cheap and underrated way of getting this data. Does anyone know this or can help me find? I’m very curious how they are getting fresh data again and again!