Integrate SSO into my web app for a client wanting mandatory Azure SSO for their staff members to acceess my web app

Question

I am new to SSO and MFA and have a client (Client X) who uses Azure / AD and wants their users to access my Web App via Azure SSO.

We would like to incorporate SSO and MFA as options for all clients in the future and let them select the type of authentication. For example, some staff members may want to use Google SSO others may not want it at all.

So I guess I have two parts.

1) Enabling SSO as an option for users and mandatory for staff belonging to Client X.

2) Guide on implementing the Azure SSO for staff belonging to Client X

I was looking at off-the-shelf paid providers like Auth0 but would rather create it in-house and have full flexibility over the solution and not be wedded to a third party and have to pay per authentication like we would have to with a provider like Auth0.

Any assistance or starting points would be greatly appreciated.

Kind regards,
David