Each help desk has its own General settings, under Configuration → General: branding, the primary domain for its customer portal, locale, and business hours.
Business hours aren't just cosmetic — they feed your SLA targets. First-response and resolution clocks only run during the hours you actually operate, so a ticket that arrives on Friday evening isn't unfairly counted as breaching over the weekend if you're closed.
Because branding and domain are set per help desk, each brand's customer portal can look like its own product. Set these up early so your portal and customer-facing emails are correct from the first ticket.