OP mentioned a 600W charger. Sure there’s no danger in plugging into a USB port but 600W of USB ports would look dumb. They also mentioned a box fan.

USB ports are fine for USB things and those provided by the charger may be slightly more efficient than using one of the AC plugs and a separate phone charger but were taking marginal gains on a tiny load that runs for a small amount of time. On a large AC battery system, the USB ports are just as likely to use an off-the-shelf AC/DC converter like most chargers as they are to use a DC/DC converter built for their operating voltage. They’re also limited in power output to whatever the battery manufacturer decided, which is probably not a lot.

A hypothetical person with infinite time, knowledge, and money, could create a whole DC ecosystem yes, but someone like OP posting for an ELI5 on the difference between AC and DC needs an AC system 99.99997% of the time. If it comes with extra USB ports, whatever, they’re fine I guess. But don’t try and run a large box fan on them.