Configuration.yaml is the main location for Home Assistant for storing configurations. Edits such as adding a new entry requires a full HASS restart. Where as editing other parts can be refresh by hot-reload.
Configuration can be split
https://www.home-assistant.io/docs/configuration/splitting_configuration/
For example, with !include_dir_merge_list in configuration.yaml
mqtt: !include_dir_merge_list mqtt/Now the mqtt configurations can be placed in a folder ./mqtt relative to config and and with files teslamate.yaml and sonoff.yaml which consists list of acceptable mqtt entities. Everything will be merged together as a single list.