Difference between VPS and cloud server

Different providers can call the same in different ways. Digital Ocean's Cloud server is equivalent (same type of product) to an OVH VPS or Amazon EC2 instance, virtual machines are hosted in its infrastructure.

The special infrastructure can be understood as a very large computer (actually sets of CPUs, RAM and disks) that the orchestration software presents to the administrator as a large set of resources that divides into virtual machines and presents, in turn, the clients. As if you install Virtualbox on a computer, you generate two virtual machines, give it 2 IP's and charge someone for accessing and using those virtual machines.

I think they do not add a swap partition because, depending on the software that is going to be used, it is not always advisable to use it, especially if you want to fine-tune the performance. Also there are usually guides from the providers on how to add swap in their VPS.

When you have a VPS server you get the same service from the server where several services are exploited, now the cloud is more robust in its infrastructure and that is why it is more ergonomic in virtual environments.

I hope the brief explanation will work.

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Exactly, the cloud at the infrastructure level is much better because there are several clusters, an example is that for x and reason you have 3 services to VPS but the server will know that it is the fault of the same, however if you have those same 3 Services in a Cloud will be more likely than just one to present problems, since providers usually place services in different clusters.