Main sections:
"queryPlanner", "executionStats", "allPlansExecution", and "serverInfo".

Key fields from the most important stages, like IXSCAN and FETCH + some mapping of old fields to new fields
Explain tree structure of queryPlanner.winningPlan and executionStats.executionStats.

/reference/method/cursor.explain()
/core/query-plans
/tutorial/analyze-query-plan
/core/query-optimization
/tutorial/measure-index-use
/reference/operator/meta/explain
/tutorial/evaluate-operation-performance