To people working in corporate or in startups. What exactly is corporate credit card and does everyone in the company gets access to use it?

In some companies, it used to be provided to everyone, but in most companies, Not everyone gets it. Usually very senior level folks get it, or you get it if you get sent on short onsite assignments. It is primarily meant only for spending the office related expenses.

Can vary company to company. Some senior leadership/all FTE get it. You are supposed to use it for official purposes only.

While travel ticket booking during business related travel, while paying for food (maybe during client dinners or so). Expensing certification/courses, etc. There is usually High credit limit.