The documentation on the merge policy is here.
The merge policy is the Tiered one.
It is not usually required to tweak settings which are not documented as they're usually necessary for special edge use cases (e.g. heavy delete use cases).
Other advanced settings are at:
- https://github.com/elastic/elasticsearch/blob/7b34e22890e15b7480689b216c48487c87385978/server/src/main/java/org/elasticsearch/index/MergeSchedulerConfig.java#L56
- https://github.com/elastic/elasticsearch/blob/7b34e22890e15b7480689b216c48487c87385978/server/src/main/java/org/elasticsearch/index/MergePolicyConfig.java#L49