Add a shared Mailbox to Outlook Mobile: https://support.microsoft.com/en-gb/office/add-a-shared-mailbox-to-outlook-mobile-f866242c-81b2-472e-8776-6c49c5473c9f
Add shared mailbox to Webmail: https://support.microsoft.com/en-us/office/open-and-use-a-shared-mailbox-in-outlook-web-app-bc127866-42be-4de7-92ae-1ef2f787fd5c#:~:text=Sign%20in%20to%20your%20account%20in%20Outlook%20Web%20App.,you%2C%20and%20then%20select%20Add.