Github actions: Run step / job in a workflow if changes happen in specific folder

You could use the paths-filter custom action with if conditions at the jobs or step levels, using a setup job as preliminary to check if your specific path has been updated, saving the result as an output.

Here is an example

name: Paths Filter Example

on: [push, workflow_dispatch]

jobs:
  paths-filter:
    runs-on: ubuntu-latest
    outputs:
      output1: ${{ steps.filter.outputs.workflows }}
    steps:
    - uses: actions/checkout@v2
    - uses: dorny/paths-filter@v2
      id: filter
      with:
        filters: |
          workflows:
            - '.github/workflows/**'
    # run only if 'workflows' files were changed
    - name: workflow tests
      if: steps.filter.outputs.workflows == 'true'
      run: echo "Workflow file"

    # run only if not 'workflows' files were changed
    - name: not workflow tests
      if: steps.filter.outputs.workflows != 'true'
      run: echo "NOT workflow file"

  next-job:
    runs-on: ubuntu-latest
    # Wait from the paths-filter to be completed before starting next-job
    needs: paths-filter
    if: needs.paths-filter.outputs.output1 == 'true'
    steps:
      ...

That way, you could have something like this in your jobs: A --> B or A --> C depending on the path that has been updated.

Yes: https://docs.github.com/en/actions/learn-github-actions/events-that-trigger-workflows#registry_package

This is the syntax:

on:
  push:
    paths:
      - 'a/**'
      - 'b/**'