Al Hobbs

To setup the VPC endpoint to access the S3 you use the routing table.

Even if you use VPC it's still the public cloud. It's on the same physical server as another person's resources.
Not sure the reason they made it like that.
Maybe to simplify the service.

one is certainly using VPC and never without VPC?

yes. if you create an EC2 instance you can see the VPC it is in

resources just using public cloud (without VPC)?

AWS has many services available and you would have to look it up

S3 is not within the VPC but is communicated with through a VPC endpoint.

When you use you resources in AWS there is a VPC that the resources are in. You communicate with those resources through that VPC. The VPC is what allows your resources to have it's own public / private IPs.

EC2, Glue, S3 require a VPC otherwise they couldn't be used.

When you use aws, the resources you create like EC2 have a default VPC. The VPC allows the instances or other resources to have their own network even though they are in the same server.

So you should combine 1 & 2

sure.
you could mention AWS Outpost.
https://aws.amazon.com/outposts/servers/?nc=sn&loc=0
It allows you to buy a server from amazon and put it on premise but it still has to connect to AWS.

Have you tried debugging it?

If you are using AWS or Azure then you are using a public cloud.

Where are you handling the POST request?

if the company has it's own 'cloud' infrastructure, then that would likely mean they also have some kind of automated software for deploying applications that they built or licensed.  If they have their own servers that run things then that would be called on-premise.

In this example you mean backup in some cloud like AWS/Azure?

Yes.  They would have their own setup on-premise then if more resources were needed their backup in AWS or Azure would also be used.

The jar needs the javafx jars on the module path.

Have you verified that the ID is correctly being sent to the server?

Correct.  Using the cloud means you are using another company's infrastructure and services. They could use them together. For example, the company could mainly use it's own servers, but have a copy in the cloud as a backup or to handle an unusual traffic load.

it looks like you are submitting the form.  You aren't trying to do that?

That book looks awesome actually.