the filebeat-7.3.0-* is an index pattern, this will match all the indexes that starts with filebeat-7.3.0- and, depending on your filebeat configuration, you can have one index for every day or one index a month.
Let me first ask you few things:
- can you tell me a bit more about your use case? reindexing is fine if you just want to snapshot the data available on the cluster A to the cluster B (this can solve your import/export issue but this will allows you to visualize only the subset of logs reindexes, no any new logs)
- why do you want to export and import a dashboard into a different kibana environment?