With the newer versions of Outlook, the standard buttons (for Mail, Calender, Tasks etc.) has been moved to the left side, instead of below the mailbox-contents
Left-Right screen estate is precious (for mail preview) while Up-Down is more allowable (at least to me.
https://learn.microsoft.com/en-us/answers/questions/4617774/can-we-move-the-toolbar-back-to-bottom-of-left-sid
It could seem like only the “Classic” Outlook supports this (Unconfirmed)
To my understanding you’re not using the new Outlook , I have done some research on your inquiry and concluded that Navigation bar location can be adjusted to the bottom by applying the following steps:
Open the Outlook desktop app.
Click File, then Options.
In the window that opens, select Advanced.
Under the Outlook Panes section, uncheck the box that says “Show apps in Outlook.”
Restart Outlook and your navigation bar would be relocated at the bottom