You would have seen those black blobs in PCBs in those cheap mass-manufactured toys. It’s called a Chip on Board(COB).
COBs are placed on the PCBs via Wire Bonding. Wire bonding is a solid-phase welding process that joins a thin wire and pad surface. Now a COB is usually a chip with just the bare silicon die with exposed metal pads. This means that the plastic packaging that you find in most ICs which makes it a packaging style of QFN, TFQP, etc is missing. The metal pads on the die are connected via wire bonding directly to the PCB pads. Pour some black epoxy for strength and there you have a full COB on a PCB. It always need not be black epoxy, for SMD RGB LEDs if you look carefully you will see wire bonding used to connect the die to the package, and then the clear epoxy is poured to act as a lens to focus light.
Wire bonding can be done using Gold, Copper, or Aluminium wires based on requirements with Gold having good conductivity, resistance to oxidation, and good malleability to create strong bonds. But they are expensive. Copper or Aluminium is used when price is a concern. The wires are extremely thin with diameters in 10s of micrometers. To bond the wire on the pad you have different techniques like thermocompression bonding(using high heat and pressure to bond), ultrasonic bonding(Ultrasonic vibration to excite the spot to bond), or thermosonic(which is a combination of the above two methods).
Now Why wirebond on PCBs? Why not just use the chip as it is with the plastic package and solder it? The answer lies in the cost. For extremely large volume manufacturing, you can get the manufacturer to just give you the bare dies at a much cheaper cost than that with a package. Now wirebonding + die is cheaper than having a package. This is the reason why you see them a lot in dirt-cheap products. Another reason is that the actual die is usually much smaller than the plastic package, so for extremely small products it makes sense to do die + wirebonding to save a few mm of space in the PCB area.
