Asynchronous Web APIs
The only difference with synchronous calls (you can say Normal API call) is
The route to all asynchronous calls contains the prefix /async, added
before /V1 of a standard synchronous endpoint.
Asynchronous and bulk web APIs will process API calls without waiting for the whole call queue to be processed. Using this approach greatly reduces the response time from the APIs while processing such API calls as bulk product integration, or customer import. This functionality will be available in both Magento 2.3 Commerce and OpenSource editions.
- GraphQL is a query language for APIs. It is one of the modern
technique to build and query API.
- Magento 2.3 executes GraphQL as an elective API endpoint in addition
to REST API and SOAP API.
I hope this will help. I tried to cover all your question query