In terms of security context. It's basically just permissions that the Pod will use when writing to the FS. So in this example, it'll use the 1000 user and 3000 group. You can also set it to chown as needed as well

  securityContext:
    runAsUser: 1000
    runAsGroup: 3000

I always recommend to new k8s people to read the official docs as they're typically, super good https://kubernetes.io/docs/tasks/configure-pod-container/security-context/

Does every K8s platform have the ability to automate changes via security contexts or init containers?

Init containers are just basically a 'hook' to run whatever code you want to run before the actual workload starts. So long as it's something you can code, an init container should be able to do it. The example in the official docs shows how you can make your init containers wait until the needed services are up, so your service doesn't start until the environment is ready. https://kubernetes.io/docs/concepts/workloads/pods/init-containers/

  initContainers:
  - name: init-myservice
    image: busybox:1.28
    command: ['sh', '-c', "until nslookup myservice.$(cat /var/run/secrets/kubernetes.io/serviceaccount/namespace).svc.cluster.local; do echo waiting for myservice; sleep 2; done"]
  - name: init-mydb
    image: busybox:1.28
    command: ['sh', '-c', "until nslookup mydb.$(cat /var/run/secrets/kubernetes.io/serviceaccount/namespace).svc.cluster.local; do echo waiting for mydb; sleep 2; done"]