Organize DNS service for SAAS customers

This is a first time ever when I'm working on SAAS project. I'm stocked with two questions regarding project architecture: - about DNS for users, how I can create DNS records, also how can users bind their own domain? - use one place stored backend code without making copies for any new user.

Project backend is being developed with PHP 7.

Great example is WIX (project that helps users to create beautiful websites just in a few clicks).

After registration you get third-level domain "username.wix.com" or something like that and also you can bind your own domain name. Framework backend code is stored in one place (it surely cannot be copied every time for any new user, otherwise service-owners will lost all control on updates and have a lot of unnessesary copies).

So the question is, what should I do to implement architecture like this, backend code is stored in one place, after user registration he get third-level domain and can bind his own domain? Meanwhile all executable code is stored in one place (eg.: SSD) and connected to all users?

Thanks in advance for any information!