How to track same visitor/user across websites?

General questions
1

Hi,
I have a number of sites/blog and also ecommerce sites. All on different domains

I wanted to see if there is a way to see a unique visitor irrespective of website. So the unique identifier issues from first visit is tracked across the ecosystem of sites and web properties.

Is there a way to track the same visitor across websites?

And then as a user ID once he/she gives their email via form or as a customer.

Then result is information is about an individual person then websites etc.

Any help would be greatly appreciated.

Thanks
JJ

2

I ran into a plugin: Enterprise Web Analytics: Self-Hosted or Cloud - Piwik PRO

But I believe this aggregates website data not necessarily user/visitor data.

Correct?

Thanks,