The two places are a deliberate hierarchy, and understanding which wins is the whole answer to raising one person versus everyone:

The database level sets the default for every mailbox in it: Organization Configuration, Mailbox, Database Management, the database's properties, Limits tab, with the three thresholds, issue warning, prohibit send, prohibit send and receive. Change these and every mailbox in the database inheriting the default shifts together, the lever for raising everyone at once.

The mailbox level overrides the database for that one mailbox: the mailbox's properties, Mailbox Settings, Storage Quotas, where unticking Use mailbox database defaults lets you set that person's own thresholds. This is how one heavy correspondent gets a generous quota while the database default stays sensible for everyone else, the override winning wherever it is set.

The right pattern therefore: set a sensible default at the database for the organization, override individual mailboxes where roles genuinely need more and check which is in play on any given mailbox by whether Use database defaults is ticked. The PowerShell forms, Set-MailboxDatabase for the default and Set-Mailbox with the quota parameters for overrides, script the same hierarchy for bulk role based assignment.

The caution that belongs with raising quotas: bigger mailboxes mean bigger databases, longer backups, slower restores and more storage, so the increase is a real resource decision rather than a free click. Pairing quota increases with archive mailboxes, moving old mail out so the quota governs the working set, is the arrangement that raises usable capacity without inflating the primary database indefinitely, covered in the size limit thread linked alongside.