The quarterly report didn’t really get into it much, but Other (income) is usually where things like loyalty programs, credit cards, commissions, etc come in (at least the parts that aren’t specifically related to travel redemptions). Delta gets into much more detail, perhaps United will do so in their annual report.

Other (expenses) is stuff like crew hotels and ground transportation, aircraft catering, etc.

Sankey diagrams are kind of low-effort, but they’re very well suited to things that flow, like rivers, and money.