Thanks for getting in touch! So, transactions belong to the Orders resource. Check out the Order transaction docs here to see how you’d GET those transactions in REST. It’s possible via GraphQL Admin API too (docs). Hope that helps!
You can get the contact email that is associated to the order with a GET to the Orders.json endpoint in REST. Feel free to check out our documentation - there will be some example requests (in REST here) and some example queries in GraphQL here. In GraphQL it’s Order.email that exists on the Order GraphQL object. Hope that helps!