The pen contains a chemical that interacts with the paper that's always used to make these bank notes. Specifically it blackens the starch found in wood pulp, and the paper in your laser printer, photocopier, etcetera uses wood pulp because that's cheap. Bank notes use a higher quality paper and so they aren't turned black.

This forces crooks to use more expensive and traceable high quality papers for their counterfeit notes or they'll get rejected in stores and bars.

Having IDs that actually look up to anything at all is a relatively modern idea. When I was born if you suspected a passport in my country of being bogus it'd probably take a bunch of clerks several hours of physically looking through filing cabinets to check.

And where we build systems that can check often people don't. The UK government built a system which lets a driver prove to the government who they are and then get a token value back which they can give to anyone - that token can be exchanged for viewing the government records for that driver. So e.g. hire firms could insist on this token to see you're not disqualified and actually have the entitlements your physical driving license says you have.

They don't. Some of them will let you give them this token reluctantly but all prefer you give them a print out, which obviously you could just fake.