I am new in Magento and I am preparing to start learning the cms. I'd like to know which has lower learning curve, just Magento or React+Magento. (I already know reactjs)
Magento first in my opinion. The PWA Studio it's just a front end tool that uses the Magento as a backend.
But, as Ben Crook said on the comments, learning only Magento, you will have to know many other things like RequireJS, KnockoutJS, LayoutXML, PHTML, jQuery. So, to improve my answer I would say that you can both learn Magento Backend and the PWA solution at same time. You don't need to learn the standard Magento Frontend, in this case.