I came across CDK and SAM after having spent a reasonable amount of time playing with Amplify and React. My guess is that Amplify is now sufficiently developed that it enables to do pretty much all is needed for building fullstack applications. However, I don't know yet if it is worth knowing CDK and SAM also. In other words, what are the advantages and limits of these three services?
Amplify focuses on the frameworks you specify and the needed infrastructure you need for the defined softwarestack. Its a bit like Ruby-on-Rails with more features when it comes to creating the cloud resources also.
CDK has a way broader scope (and SAM a way lesser) when it comes to infrastructure. To summarize: If you go all-in with AWS, you definitely need CDK to really create all the resources. I am pretty sure that at some point in time, you want to have more resources than Amplify will create for you. To me, CDK is essential for every bigger project (when not using one of the CDK competitors).