How can I undo the changes to my apple mail outlook?

Hi,

Once again, the required mac update has made changes which I do not prefer.

I loved the simplistic one inbox display my mac mail had previously.

Now with Big Sur, it has become a chaotic mess, showing way too much on the Apple left side bar.

I do not want my sidebar to show "Mailboxes", "Favorites", "On my mac" and "Smart Mailboxes".

Which macOs was the one with the joint inbox (joint trash etc.) ? And how can I return to that?

Thanks