How can I embed a ReactJS app into my Shopify theme?

Store Design
1

I have a basic Shopify theme on my store at the moment and it is working well.

There’s some more advanced functionality I want on a specific page (i.e. a dynamic user calculator for them to calculate some complex data). This is for my customers to use, not a merchant tool. I know I can build this in ReactJS quite easily but I am wondering a few things.

  1. How can I embed this ReactJS app into a page on my Shopify store seamlessly? Is an iframe the only way, or is there a way to make it more seamless?

  2. In both cases, how can I pass the current user’s login details to my React app to pull up their history if needed?

Thanks!