For an MCU, where do you find register definitions versus electrical limits?

They're in different documents, and knowing which is which saves a lot of time. The register-level programming detail, the full register map with addresses and bit-field definitions, and the functional description of how each peripheral behaves, lives in the reference manual, which is typically shared across a whole family of parts. The electrical and timing characteristics, absolute maximum ratings, recommended operating conditions, DC characteristics like voltage thresholds and pin currents, and timing specs and diagrams, along with the pinout, package, and ordering information, live in the datasheet, which is specific to the individual part number. So if you're writing a driver and need to know which bit enables a peripheral or the address of a status register, you go to the reference manual; if you need to know the maximum current a pin can sink, whether a pin is 5 V tolerant, the supply voltage range, or a SPI setup/hold time, you go to the datasheet. (For a standalone sensor or simple IC, this is usually combined into one datasheet.) There's also the errata for known bugs and application notes for guidance. The common beginner inefficiency is hunting for register bits in the datasheet or electrical limits in the reference manual; recognizing the split, reference manual = how to program it, datasheet = its limits and pinout, lets you jump to the right document immediately, and you frequently cross-reference both (and the errata) for a single task.