AD B2C Custom Policy - separate SignUp/SignIn flow

Question

Hi,
I would like to create an AD B2C custom policy to meet the following business requirements.
Business Requirement
The product sold to customers has a unique product identification number and the customer registers an email Id while buying a product from the dealer. These customer details (product identification number, registered mail Id, and other details) are then stored in the manufactures database. The smartphone application to be developed for end customers to manage/view the health status of the product requires the customer to sign up as follows -
Step 1
The application provides an interface to input "Product Identification Code" and "Registered email ID". Upon submitting these details, the system should check if the entered "Product Identification Code" and "Registered email ID" exist in the manufacturer's database.
--> I could do this by implementing a REST API and custom email verification method to generate and send OTP to registered email ID.

Step 2
The customer enters the OTP received on the registered email Id for verification.

Step 3
Post OTP verification, customers can sign up using a local account or social account.

This step basically links customers social profile with product and create user AD B2C.

Lastly, Sign In

What I could achieve till now -

Using REST API for validating "Product Identification Code" and "Registered email ID"
Sending otp to registered email Id using sendInBlue
OTP Verification
Local account creation
Now, I want to have social account (google and facebook) option as mentioned in Step-3.