What are the benefits of "custom" ChatGPT

Hi!
I’m pretty new in OpenAI community and I want to know or where I can read about Custom ChatGPT’s? I found a link that redirects to ChatGPT but seems like a custom ChatGPT.

Link: https://chat.openai.com/g/g-8YgEfR7ig-cortexjs-assistant

What’s up with this? Can I “learn” an assistant or “fine-tune” a model and then upload it as a Custom ChatGPT? Do I get paid for this?

Thansk for all the explanations.
Luka