Gitlab CI - Multi project pipeline triggering Parent Child pipelines

Question

Lets suppose Pipeline A is a multi-project pipeline. and Pipeline B is Parent-child Pipeline.

Pipeline B is triggering jobs based on the changes inside each of its subfolders.

Structure of Project B:

.gitlab-ci.yml (here, referred as Pipeline B)

subfolder1/.gitlab-ci.yml

subfolder2/.gitlab-ci.yml

stages:
  - setup
    
subfolder1-job:
  stage: setup
  trigger:
    include: subfolder1/.gitlab-ci.yml
    strategy: depend
  rules:
    - if: $CI_COMMIT_BRANCH != "main"   # when the commit happens in any other branches other than main
      changes:
        compare_to: 'refs/heads/main'
        paths:
        - subfolder1/*
    - if: '$CI_COMMIT_TAG || $CI_MERGE_REQUEST' #it should not run in any MR/TAG
      when: never
    - if: $CI_PIPELINE_SOURCE == "pipeline"     # **this is the block which needs to be developed.**
     

This code works perfectly fine when there is a change in the subfolders and on any branches other than main.

Now the multi-project pipeline must deploy only those changed subfolder related changes into PRODUCTION from main branch. It should only trigger those subfolder/.gitlab-ci.yml files inside the parent child pipelines. Also, The trigger should be from the Pipeline A.

Pipeline A code:

trigger_job:
  stage: prod
  inherit:
    variables: true
  trigger:
    project: project/projectB
    branch: main
    strategy: depend

What happens now : When the pipeline A triggers pipeline B , it is triggering all the jobs (not just the changed ones) similar to a "RUN PIPELINE" manual event.

What I want to achieve is Pipeline A -> triggers pipeline B -> In pipeline B for this trigger event it should look for changes in the subfolders in that main branch from a previous tag(passed from pipeline A).

How to combine the below into a condition ?

if: $CI_PIPELINE_SOURCE == "pipeline"  #it identifies that it is triggered from pipeline

compare_to: 'refs/tags/$PREV_TAG'

and then check

changes: subfolder1/*

all under the Pipeline B's job subfolder1-job !

If you think there is a better way to achieve this, please feel free to write back. Also if you think a way to deploy from pipeline B itself(not after the MR success - as this is a manual prod deployment)

Thanks for reading this long message.