What do I need to do to start building an Instagram bot for others (different Instagram accounts)?

You’ll need 1 Manychat account per IG account.

To connect them, you either have access to the IG accounts, or you get your customers to connect their IG to Manychat and then add you as an Admin on the Manychat account. I prefer option 2, and I usually help my clients get connected on a Zoom onboarding call.

I have done what Gustavo Boregio said and I’ve also sold Manychat chatbot templates, you can create a template in your own Manychat account and sell it with a short tutorial/course that shows how to set it up.  here's a link that shows how to set up templates! https://support.manychat.com/support/solutions/articles/36000024524-how-to-create-templates

For the option 1: “To connect them, you either have access to the IG accounts” , do I need to have its credentials or just all permission to if and fb page?

thanks

As of July 13th, only the credentials should be enough:

I haven't tried it yet.