I think the easiest way to understand whats going on under the hood when you create kubernetes resources using imperative commands (versus declarative approach by writing and applying yaml definition files) is to run a simple example with 2 additional flags:

--dry-run

and

--output yaml

Names of these flags are rather self-explanatory so I think there is no further need for comment explaining what they do. You can simply try out the below examples and you'll see the effect:

kubectl run nginx-example --image=nginx:latest --port=80 --dry-run --output yaml

As you can see it produces the appropriate yaml manifest without applying it and creating actual deployment:

apiVersion: apps/v1beta1
kind: Deployment
metadata:
  creationTimestamp: null
  labels:
    run: nginx-example
  name: nginx-example
spec:
  replicas: 1
  selector:
    matchLabels:
      run: nginx-example
  strategy: {}
  template:
    metadata:
      creationTimestamp: null
      labels:
        run: nginx-example
    spec:
      containers:
      - image: nginx:latest
        name: nginx-example
        ports:
        - containerPort: 80
        resources: {}
status: {}

Same with expose command:

kubectl expose deployment nginx-example --type=NodePort --dry-run --output yaml

produces the following output:

apiVersion: v1
kind: Service
metadata:
  creationTimestamp: null
  labels:
    run: nginx-example
  name: nginx-example
spec:
  ports:
  - port: 80
    protocol: TCP
    targetPort: 80
  selector:
    run: nginx-example
  type: NodePort
status:
  loadBalancer: {}

And now the coolest part. You can use simple output redirection:

kubectl run nginx-example --image=nginx:latest --port=80 --dry-run --output yaml > nginx-example-deployment.yaml

kubectl expose deployment nginx-example --type=NodePort --dry-run --output yaml > nginx-example-nodeport-service.yaml

to save generated Deployment and NodePort Service definitions so you can further modify them if needed and apply using either kubectl apply -f filename.yaml or kubectl create -f filename.yaml.

Btw. kubectl run and kubectl expose are generator-based commands and as you may have noticed when creating your deployment (as you probably got the message: kubectl run --generator=deployment/apps.v1beta1 is DEPRECATED and will be removed in a future version. Use kubectl create instead.) they use --generator flag. If you don't specify it explicitly it gets the default value which for kubectl run is --generator=deployment/apps.v1beta1 so by default it creates a Deployment. But you can modify it by providing --generator=run-pod/v1 nginx-example and instead of Deployment it will create a single Pod. When we go back to our previous example it may look like this:

kubectl run --generator=run-pod/v1 nginx-example --image=nginx:latest --port=80 --dry-run --output yaml

I hope this answered your question and clarified a bit the mechanism of creating kubernetes resources using imperative commands.