How to organize the infrastructure of many applications?

Question

Software design

First, I will describe the situation:
I have clients of a legal entity who have access to our online system. Each legal entity has employees. Employees have different roles, as a result, they have different access in our system. We also have partners who have access to their clients' accounts. We have access to the accounts of all clients and partners. Plus, we are gradually expanding the range of opportunities available to customers and partners, complicating the system, which is quite monolithic.
Now about what you want to get:
Divide the system into "applications": configuring subsystems, reporting, working with clients, etc. As a result, it turns out that there should be a central service with customer and partner accounts, with user data (logins and passwords for authorization), which will just do the authorization and give data about what services are available to customers according to their contracts. And the applications themselves, which will authorize users and provide access to one or another of their resources, according to the information received about the user's access level.
Now to the essence of the question: where, what can you read about it? any solutions can be used from those available on the market and in open-source? especially, it seems to me, for the central authorization service and access rights.