Is the "Backend" for new Headless Shopify the same as for traditional Shopify "Backend" ?

[elicommerce2018]

I intend to develop in Headless Shopify platform and would greatly appreciate if I could get answers to the following questions.  Please answer as many questions as you are comfortable with.

1.  Is the "Backend" for the new Headless Shopify the same as the traditional Shopify "Backend"?  If not, where on the partner.shopify we would create that backend?

2.  Does the new Headless Shopify have to built in Shopify+ platform or the Classic Shopify environment can be used?

3.  Likewise, does Hydrogen and Oxygen work only on Shopify+ or it can be used to develop Frontend for Classic Shopify sites as well?

4.  If the the answer to the above question is "No", what CMS tools are recommended to be used as instead?