Comment by Neywiny

Usually (not always), something like a Teensy or a Pi Pico or an Arduino is treated like a development board for prototyping.

A person builds out their circuit using hardware they can solder/wire-up by hand on a workbench, maybe even with relatively-giant solderless breadboards, to prove the concept and the general design.

And a dev board can be great for spinning a few prototypes. It's quick to get started (code can begin being tested on-chip after just plugging in a USB cable), and to try different things and to make (and correct!) mistakes. (Blow up a Teensy? No worries; just grab another from the drawer, try not to make that same mistake again, and keep moving -- no esoteric soldering required)

But when the design is finished-enough and it becomes time to spin up custom-built PCBs for a final product that will be sold, a separate dev board like a Teensy tends to lose much of its initial charm.

Instead, it's more-typical just put the microcontroller IC plus whatever supporting hardware is necessary for the overall device's actual functions on the main board. Don't need USB, or an Ethernet PHY, an LED, a button, or a separate voltage regulator? Want more or less flash? When including the MCU on a board of one's own design instead of a kitchen-sink dev board, one is empowered to use exactly the parts that are required.

This can save a substantial amount of space and greatly improve the flexibility of the layout, while also improving mechanical and electrical robustness by having fewer connections between the MCU and the world around it. Plus, fewer parts tend to be less costly than more parts are.

(But again, it's not always done this way. This camera from the submarine is an example of one instance where the whole dev board was put inside of a finished product. Sometimes that's a good idea, and sometimes it isn't. I'm not attempting to suggest that it was or was not a good move in this instance.)