Domains are a construct in OMOP to organize vocabularies to standard concepts. The OMOP Vocabulary group determines the mappings. If there are issues that the community finds with the mappings, people either post them in the forums or in the GitHub repo. You can find out a little more about domains here.
As for concepts mapping to multiple domains, the general rule is to create a row for each mapping in the appropriate domain table. Not all ICD codes, as you’ve seen, are in the condition domain. @Patrick_Ryan discusses it a little here.