Why is it possible to run Go tests and builds inside CI environments without having to install the dependencies first?

I have a Go project with a Makefile

test:
    @go test -cover ./...

and a mod file

module path/to/repo

go 1.19

require github.com/go-chi/chi/v5 v5.0.8

I created a Github action sample to run tests on a Github PR

name: QA on pull request

on: pull_request

jobs:
  run-tests:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout repository
        uses: actions/checkout@v3
        with:
          fetch-depth: 0

      - name: Setup Go
        uses: actions/setup-go@v3
        with:
          go-version: 1.19

      - name: Run tests
        run: make test

I would like to know why this workflow is working without a install dependencies step. The project itself is using external dependencies and I think there should be a step that runs go get ./...

Does Go install them under the hood if not present? Or does the action actions/setup-go@v3 install the dependencies?

According to go test docs (or you may run go help test locally to read its description):

> 'Go test' recompiles each package along with any files with names matching the file pattern "*_test.go".

It also installs all the dependencies; so, it happens when the action does go test. You can probably observe it in the logs.

actions/setup-go@v3 doesn't depend on the code itself. It just sets up the go environment you ask for. In your setup, if you swap the setup-go and checkout, it still works.