if you want the transaction ID in your dashboard visualization you will have to configure it after the logs are populated in Kibana (any tutorial online will enable you to do that)
But if you are looking at the custom log fields you will have to use -
a. Add log field
b. Feed data to the log field by logging activity
c. Remove log field when you dont need it (very important as if you forget to remove it every other log message will have the field as empty giving you wrong results in Kibana dashboard)
Coming to the association question, you can run a workflow locally (in developer mode) and double click the log message in the output panel and check for all the fields associated with that particular log message and you can see all the default log fields being populated in that window. hence the transaction ID association. Also in REF you can find the Trans ID and Queue name are added as log fields and you can choose to remove it.
Log message contains fields and specific content is a custom object to retrieve queue items/ values under headers, nothing special about it.
Hope this helps, revert if you have further queries.