What do all these AI Agent startups actually do?

Every day I open the news, this AI Agent startup raised 60 million, this one valued at 3 billion, and more. What do they actually innovate? Are they just using existing opensource LLMs, refining, and selling them as a product with an interface? I’m new so I just want to understand.

Also what’s stopping openAI from building a platform for every company to make their own agents in house? What will these startups do since they are not making the LLMs?