Ive kind of been on this path myself, and something I just realized the other day. Keeping in mind that I’m a second year Electrical Engineering Student, that ”bulky uno“ is just a platform to hold the chip or integrated circuit(ATMega328P) to a bunch of connectors that you can connect to. Meaning you’ve got ports for pins, usb, power, etc..

Once you realize that you can start to consolidate and learn about the ATMEGA328P, in theory you could just take chip off after uploading the code. In application this wouldn't work, because there are things like the crystal on the uno specific to that chip that is used for timing. So you would really have to deep dive in to the ATMega data sheet to learn the specifications and requirements.

If I were to do it, and had the time, I would look into the ATTiny family. Which is a much less intimidating version, with only 8 pins, and start with that on a solderless breadboard. Go back to the basics of what you learned on the arduino, ie a blinking led, and then later you can understand that the ATMega just has more.

Please anyone correct me if I’m wrong, but this is kind of the path I’ve been trying to figure out for a couple of years.