Frontend and backend API, two services (components) – it’s already some kind of microservice architecture. The questions are how big your components, what kind of logic they have, will you have benefits if you split some logic to different services?
Per microservice architecture every service (component) of your system should have some dedicated logic (domain), solves some related problems, persists data to its own data-store, can be developed and deployed separately. In some cases, data-store can be shared between services.
So, the goal of splitting logic into different services is making your application easier to develop, maintenance, support and understand. Too many small services can bring a lot of overheads. To create a service, you need to spend additional development time, a service is also a deployment item, communication between services has network overheads. So, you should careful consider all pros and cons of splitting logic into services. Some balance should be found.
Going back to the question if "Users" and "Purchase History" are totally different, they don’t have common logic, can be stored in different databases and both are complicated enough, so you can split them into two services. The same about UI parts. The main thing is that splitting should bring you benefits - not overheads.
About using rest - it’s up to you, rest architecture is not required for microservice architecture but very often they are used together. Rest is about design of your services, how they expose API and so on.