One can get the access token using api-key?
This is an extremely common place to get confused so don’t feel bad (it took me months to get what was going on ).
The api key is the authentication method for the legacy XML-RPC. The new api can use EITHER the api key method or OAuth. If you wish to use the REST implementation of the api, then you must use OAuth.
So the api key is a separate authentication method and has nothing to do with getting an access token. I have a video on using OAuth to help others get over the hurdle of understanding it which I’ll share below: