> How does Buffer operate then?

Buffer documents a number of workflows and limitations in their FAQs.

E.g. for a non-professional Instagram account, the user gets a notification to manually share a post via the Instagram app.

> you can prepare your post in Buffer, receive a notification on your mobile device when it’s time to post, then tap the notification and copy your post over to the social network to finish posting.

source: https://support.buffer.com/article/658-using-notification-pu...

Postiz docs show that users can create an app on Facebook and use that key and it will auto post.

I guess using POSSE for Instagram forces you to either create a personal app on Facebook which is not easy or make your Instagram account a business account.