Shared Channels will be rolled out as a public preview in the coming weeks. This is a new channel type that will be added to the existing standard and private channels. Shared Channels allow collaboration with external users without the need for the user to switch tenants:
Shared Channels are based on Azure-AD B2B Direct Connect. This Azure-AD feature enables collaboration with external users without the need to add these users as B2B guests in Azure-AD. To use shared channels between tenants, both sides must configure B2B Direct Connect policies because the feature is disabled by default. Administrators can grant, restrict, or revoke access for external users on an individual, group, or client level:
More details about Cross-tenant access settings: Cross-tenant access settings in Azure AD
A separate SharePoint SiteCollection is created for each shared channel, just as for each private channel. Conditinal Access Policies for SharePoint therefore also affect Shared Channels and the co-authoring of documents in these channels.