How to update users table including relation tables with entity?
Long answer short: You can't save them at once and have to synchronize the user fields manually.

Users are fieldable entities and field data is stored in different tables. So in your case you'd have to create all fields you like to synchronize in all your sites and you need to read them from your base site's user entity field by field and write them to the according tables of your target sites.

A very error prone approach on database level (not mentioning the complexity with a growing number of attributes/client sites).

I strongly suggest a less low-level solution. E.g. by utilizing Drupal's REST services, considering OAuth2/OpenID server/client, LDAP or SAML for user authentication. (Just some keywords to use Google with; there are some contrib D8 modules in heavy development for all of these approaches.)