You need to check several things in the case of services:
What user the service runs under, and what groups that user belongs to, as that defines what permissions the service will have. Check this. Spoiler: use tools like top and some filtering.
If you’re interested in things like “what permissions a file created by service A would have?”, then you need to research umask (User file creation MASK). There may be some umask sentences in the service definition scripts or files, or in the process’ home directory if its user had one; check this.