Syntax:
JSON_MERGE_PRESERVE(json_doc, json_doc[, json_doc] ...)

Merges two or more JSON documents and returns the merged result.
Returns NULL if any argument is NULL. An error occurs if any argument
is not a valid JSON document.

Merging takes place according to the following rules. For additional
information, see
https://dev.mysql.com/doc/refman/8.4/en/json.html#json-normalization.

o Adjacent arrays are merged to a single array.

o Adjacent objects are merged to a single object.

o A scalar value is autowrapped as an array and merged as an array.

o An adjacent array and object are merged by autowrapping the object as
  an array and merging the two arrays.

URL: https://dev.mysql.com/doc/refman/8.4/en/json-modification-functions.html

