Thea Flowers (Stargirl, @theavalkyrie) creates open source and open hardware craft synthesizers that use Circuit Python for customization. She also writes about the internals of the SAMD21.

Thea's synthesizer modules are found at Winterbloom, including Castor & Pollux and the Big Honking Button. It is all open source hardware so you can find code and schematics on Thea's github site: github.com/theacodes

Thea's site is thea.codes. You can find her blog there with deeply technical and detailed posts such as The most thoroughly commented linker script (probably), The Design of the Roland Juno oscillators, and Understanding the SAMD21 Clocks.

For more information about the Eurorack, listen to Embedded 356: Deceive and Manipulate You with Leonardo Laguna Ruiz of Vult.