Answer by Jay Lee for Query string for messages list returns inconsistent results compared to web interface

1. Yes, message body is searched.
2. Try: "from:example.com OR to:example.com"
3. No, Gmail UI and API search is not case-sensitive.

Be aware that service.users().threads().list() would be more consistent with Gmail UI search assuming the user has conversations enabled which is the Gmail UI default.

in:anywhere expands the search to Trash and Spam which is not normally included. Archived messages are normally included.