Both replica set and deployment have the attribute replica: 3, what's the difference between deployment and replica set? Does deployment work via replica set under the hood?

configuration of deployment

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-deployment
  labels:
    my-label: my-value
spec:
  replicas: 3
  selector:
    matchLabels:
      my-label: my-value
  template:
    metadata:
      labels:
        my-label: my-value
    spec:
      containers:
        - name: app-container
          image: my-image:latest

configuration of replica set

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: my-replicaset
  labels:
    my-label: my-value
spec:
  replicas: 3
  selector:
    matchLabels:
      my-label: my-value
  template:
    metadata:
      labels:
        my-label: my-value
    spec:
      containers:
        - name: app-container
          image: my-image:latest

Kubernetes Documentation

When to use a ReplicaSet

A ReplicaSet ensures that a specified number of pod replicas are running at any given time. However, Deployment is a higher-level concept that manages ReplicaSets and provides declarative updates to Pods along with a lot of other useful features. Therefore, we recommend using Deployments instead of directly using ReplicaSets, unless you require custom update orchestration or don't require updates at all.

This actually means that you may never need to manipulate ReplicaSet objects: use a Deployment instead, and define your application in the spec section.