Please see Using git with Angular repositories for details about how we maintain a linear commit history, and the rules for committing.
As a contributor, just read the instructions in CONTRIBUTING.md and send a pull request. Someone with committer access will do the rest.
Change approvals in our monorepo are managed via pullapprove.com and are configured via the .pullapprove.yaml file.
Once a change has all the approvals either the last approver or the PR author (if PR author has the project collaborator status) should mark the PR with "PR: merge" label. This signals to the caretaker that the PR should be merged.
See this explanation.