I would like to create an ERP website which can serve with difference companies. The website will have the following functionality

• Product Listing & Product Detail
• Shopping Cart & Online Payment
• Shipping Address & Delivery Tracking Status
• Sales Order, Purchase Order & Invoice Listing
• Login By difference Roles e.g. Sales Representative, Customer and Administrator

It is an ERP website (B2C and B2B) and it is used by difference companies and vendors.

Developed using Angular, ASP.NET Core Web API and SQL Server.

For example, each companies will have slightly difference in the product listing page

Example 1) ProductListing.aspx contains many if-then-else statement to serve for difference companies in the same pages

if CompanyA then
   do something

if CompanyB then
   do something

Example 2) Duplicated the codes with difference files, it aims for separate difference business rule and logic

ProductListing_CompanyA.aspx
ProductListing_CompanyB.aspx
ProductListing_CompanyC.aspx

Example 3) ProductListing.aspx but include/embedded difference components and each components will contains difference business rule and logic of the companies. But if-then-else statement still appear everywhere in the page

Any design patterns and best practices to improve the site to be reusable, readable, sustainable, extendable and maintainable ?