Going to try and assist here. I am taking a lot of leaps as I don't know any details of your environment other than the query above. Starting with some semantics as I want to get the terms correct and make sure we are talking about the same thing(s).
An email alias is a second address added to the primary address. For example, if primary Exchange (and I assume you mean O365) ID is blonglastname@domain.tld, a user may create an alias on the account of bob@domain.tld. Emails sent to either blonglastname@domain.tld or bob@domain.tld deliver to the same mailbox. Aliases are a process handled on the server and the client, either Mail.app or Outlook do not need any special configuration to participate.
A shared mailbox is effectively a full mailbox but it does not require a license (unless the mailbox size exceeds 50 GB), does not have a password, and grants access via delegation, thus allowing multiple people to have access to the mailbox using their own credentials. In the Exchange Online admin interface, the Shared Mailbox settings are under the Teams & Groups category. A common example of a shared mailbox would be something like sales@domain.tld or support@domain.tld. These types of mailboxes are intended to have multiple people watching them and allow a buffer to customers by concealing individual user emails. It also keeps the mail in an independent container allowing the smooth addition or removal of users with no data migration.
This leads to Mail.app and what it is lacking. Historically, if you wanted to add a shared mailbox, you needed to have IMAP enabled on the O365 tenant and you used a goofy workaround to add a shared mailbox or a delegated mailbox. I suspect your O365 tenant has enforced modern auth and in the process has shutdown the IMAP service. Generally, this is a good move related to security. However, it is a bad move for workflow as you are now unable to add shared mailboxes to Mail.app (or at least I don't know of another way and will admit that I've not starting the investigation on Sequoia yet to see if Mail.app has new tricks).
Now, is the above conditions are true, the normal advice I would give is to stop trying to fit the square peg of Mail.app in the round hole of O365 accounts. Just switch to Outlook and you will have access to all of the services features. Alternatively, if you want to continue to use Mail.app, then I would suggest using Outlook web for accessing your shared mailbox. Apple has very inconsistent support for delegation in the groupware tools. Calendar has a delegation tab but mail and contacts do not. And we will not even get into the limitation of the iOS versions.
Alternatively, you could reenable IMAP on your O365 tenant. This is generally not recommended as IMAP and POP can be attacked without triggering MFA requests. You are using multi factor on your account, right? The general guidance today is to lockdown O365 so all legacy services are disable and every account uses the Microsoft Authenticator app for MFA.
Ok, hope I did not waste your time and this is a good summary of what is happening in your environment.
