OS X 11.4

Just upgraded to OS X 11.4. When in the mail app, I noticed that now each email account is separated. I older versions I had all my various email accounts under one account.. I believe it was something like an All Mail account. Now I have to go into each separate account to read my emails. I find this rather a pain and have probably missed many emails that I thought were included in the main inbox. Does anyone know how I can get this back so that I don't have to go into each accounts inbox to read my emails?