Why can backup plugins run while the site is live?

WP backup plugins like UpdraftPlus (and others like All-in-One WP Migration that I use, WPVivid, etc.) are designed to run while the site is live to minimize downtime and maintain user access. These plugins use techniques such as database locking (and incremental backups) to ensure data integrity. Namely, during the backup, these plugins temporarily lock tables to prevent conflicting changes, ensuring a consistent snapshot, and often they back up data in chunks, reducing the load on the server and allowing the site to remain responsive.

And if a user makes a change, the plugin typically captures this in the next backup cycle or handles it seamlessly without data loss.

Maintenance mode is not necessary, but if you want to have it - you can do it manually by using a separate plugin, if you prefer no changes during backups.

Run backups via your server. Much much much more efficient.

You could jump schedule your backup and get around the issue that way

Why would you want to do this? If a user made a change to the site, if that database table was already backed up, those changes would not be included. If it wasn’t backed up yet, then it would be included in the backup.