Previously, when adding a new user to the Outlook Mobile App on Android or iPhone, you could directly select the "Exchange" option. However, this choice has not been available since the update in early April.
To configure a new account in the Outlook Mobile App, you must enter the User Principal Name (UPN) instead of the usual email address.
For instance:
Use <alias>@domain.local
instead of <alias>@ExchangeEmail.com
After entering the UPN as the email address, you can add an Exchange account.
I hope this clarifies the process.