I am not able to figure out which build tool I need to use for CI/CD for moodle?
You write your question in a very strange way.
You do not need to use a CI/CD tool with Moodle. You can use CI/CD tools with Moodle if you want, but it is not required.
If you choose to use CI/CD, then you can use any build tool you like. Moodle is based on very standard technologies (PHPunit and Behat for automated tests, git for version control, very standard deployment for a PHP + database type application) so it should not be hard to set up your favourite build tool to do what is needed.
I know some people use Jenkins, and a few people use the build & release tools in Azure DevOps, but other things will work too.
Moodle HQ share some of the scripts they use at https://github.com/moodlehq/moodle-ci-runner and https://github.com/moodlehq/moodle-docker.
This may be relevant, too: https://docs.moodle.org/dev/Travis_integration