With enough good data (so probably not in all sectors) you can also identify people out of the system.
Sure, it's technically possible. But if you would actually do that, you run afoul of the GDPR requirements for informed consent: retroactively identifying people in a dataset requires the same consent as targeted data hoovering, so if an individual has only consented to being included in anonymized statistics that practice is sure to get flagged down as unlawful.
Yes, that's definitely possible, but humans being human I doubt it can be 100% foolproof. If I go to the grocery store every Saturday at 11am and purchase a similar set of items, you can probably single me out and assign some UID to me. However, if I unexpectedly pop in Wednesday evening to just buy a bottle of wine, it would be difficult to assign the purchase to the same UID.
With enough good data (so probably not in all sectors) you can also identify people out of the system.
There are not that many bits of entropy in (contextualized) human behavior.