multi-site conection (Sigle login for multiple sites)

There are many ways to achieve what you want. If you are looking for the "appropriate" way to structure it, I would suggest to look up SSO (or Single Sign On) which is (as a concept) what you are looking for.

You need to keep in mind how Cross-Origin Resource Sharing (CORS) works as based on your domain structure you will likely encounter issues with it. Specifically, if you have xyz.domain.com then abc.domain.com will NOT necessarily share resources with the prior (because of CORS). You can configure your domains to accept resource sharing from any number of sources but you then have to keep security in mind while you do that.

Long story short, you can achieve what you want very simply if all you are after is a proof of concept, but my advise is always to keep things correct and secure and as such I strongly recommend you look into SSO.