Yeah. There's a small set of prepackaged micro- or teeny- computer programming interfaces. Or the plug-and-play if you will. In fact, that small set of convenience products only serves a market of kids that want to play with toys. They're literally toys. Ten or fifty dollar ARM microprocessors or microcontrollers coming in a box with integrated debugging features and integrated WiFi modules. And their complementary three dollar programming link handhelds. All from off the digital Amazon.com or AliExpress shelf. The "in-band" programming interface at accessible prices and stores.

And that's fine.

It's just that for me, on the other end of the spectrum, I prefer a little bit more adventure. Some less constraints. So, I need an "out-of-band" microchip programming solution for my aims.

Outside the kid world, you're required to be more knowledgeable about the way the world really works. You learn a whole lot more with out-of-band computer modifications than if you were to just plug and play some prepackaged handheld programming device into a little chip. You get more intimate with the microchip and its internals. You get concerned about its voltages and current needs, in order to achieve a proper relationship between your curiosity and the microchip's capabilities.

I want to dig into the raw power contained and hidden in unimposing millimeter (or centimeter) wide circuits. The re-programmability of microcontrollers or teeny-tiny computers, specifically.

There is no current documented solution for that. Beyond going your own way in a very long study and practice of electronics engineering and salvaging.