If your organization uses a hybrid Exchange environment, you should use the on-premises Exchange admin center to create and manage shared mailboxes. See Create shared mailboxes in the Exchange admin center
If you're not sure if you should create a shared mailbox or a Microsoft 365 group for Outlook, see Compare groups for some guidance. Note that currently, it's not possible to migrate a shared mailbox to a Microsoft 365 group. If this is something you want, let us know by voting here.
It's easy to create shared mailboxes so a group of people can monitor and send email from a common email addresses, like firstname.lastname@example.org. When a person in the group replies to a message sent to the shared mailbox, the email appears to be from the shared mailbox, not from the individual user.
Shared mailboxes include a shared calendar. A lot of small businesses like to use the shared calendar as a place for everyone to enter their appointments. For example, if you have 3 people who do customer visits, all can use the shared calendar to enter the appointments. This is an easy way to keep everyone informed where people are.