Microsoft offers with Azure AD B2B Direct Connect for shared channels in Microsoft Teams a feature that simplifies management for collaboration with external partners in teams.
Until the release of Azure AD B2B Direct Connect, an external partner was invited to the tenant as a guest and authorized in Microsoft Teams (Azure AD B2B Collaboration). The external partner then received an email with instructions for further steps.
When looking in to the own Azure Active Directory everyone notes: each external user gets a guest account in our own Azure Active Directory. Whether the guest account is still in use or not, it will always remain in Azure Active Directory until it is manually deleted.