Sharing the same code base and one database
This is very much possible using the domain module:


  "The Domain Access project is a suite of modules that provide tools
  for running a group of affiliated sites from one Drupal installation
  and a single shared database."


Which is currently under development for Drupal 8 here:

https://github.com/agentrickard/domain

This of course assumes you are not talking about actual Drupal multi-sites which are intended to share the same codebase but always have separate databases.