As you may know, Azure AD has been allowing to generate a one time password and sent it by email for external (guest) users for some time (2019 – see https://t.co/5H7QBhEa3D)
This feature was in preview and is now GA.
It will be turn on automatically for any tenant which did not have enabled it yet on March 2021.
You can still manage it from your Azure AD – either enable it now, disable or let the default activation process.
If you already had enable the email one time password when in preview, you have nothing to do but the settings have moved (see below).
To manage the email one time password, connect to your Azure AD portal (https://aad.portal.azure.com/) or Azure portal (https://portal.azure.com/) with a global administrator account and access the Azure AD blade
Then access the External Identities\External Collaboration settings blade