This isn’t strictly true, most Big Tech companies (Google, Meta, Microsoft, etc…) will often share - and sell - data that they have collected from their users with other companies. So your statement of:
Isn’t fully correct as when you use a service, such as Google, they will share (and sell) that information to a range of different companies and entities.
This also applies to your second statement:
As all the companies you have listed (Google, Microsoft, Yahoo, etc…) will share data with each other. This is why many people who are interested in privacy try to stay clear of Big Tech.