Using modern authentication

Hi, I have a scenario that I need some suggestions on. I have a bunch of apps on oracle cloud or on-prem hosted apps or other Saas apps that are used by both internal and external users. My internal users are synced from ad to azure ad and external users are in adlds. These apps use home grown login and saml identity federation (also written in .net) to authenticate these users to either AD or ADLDS depending on their domain in the username. I would like to move them to use Azure authentication for these users. Azure SAML login works for internal users but I can't figure out how to authenticate external users. The external users are either customers or b2b users or disabled/terminated/retired employees. I need some suggestions on how to design this solution please. Thank you!!