These small packs from China come in big containers from package consolidating logistic companies, with tens of thousands of others, think Temu and Shein.
The logistics company electronically submits a list to customs of every pack, listing sender, recipient, contents, and value,
If the company has a good reputation, the whole container bypasses customs. If the occasional random inspection turns up contraband or false declarations enough times, they get a fine, and if it continues, they're suspended from being able to bypass customs. Then all their containers get inspected, which can delay shipments by months, and no Chinese company will use them to ship, costing them millions a day.
That's why, when discovered before reaching the US, UGLs get their packs returned by the shippers and they have to find new companies to use.
The new de minimus rules coming into effect require a much more detailed customs declaration (ie, not "t-shirt", but "cotton and nylon v-neck t-shirt manufactured in China" ), along with details about who it's coming from. This detailed data is necessary to charge duties and tariffs in the future, and to blacklist sketchy shippers.
Customs is spot checking these declarations much more closely now, and handing out suspensions to logistics companies that have falsely declared packages in their container. (this data comes from whoever is sending the package).
In turn, the logistic companies are getting more strict with which customers are allowed to send packages through them, because a tiny number of packs containing contraband threatens their business.