[ktownsend]

Update since I should have mentionned HW as well.

If you're looking for something to play with at the HW level, it can be overwhelming to know what to start with. Stick to Arm Cortex-M. That will get you the furthest the fastest IMHO. If I was to suggest one company -- even though they all have their strengths and weaknesses and niches, and I work regularly with several like NXP, ST, etc. -- I think Nordic Semiconductors does a very good job with their support forums, SDKs, tools, etc. BLE is also a very rewarding place to start since you can talk to your phone, laptop, etc., which is Nordic's niche.

A development board like the nRF5340 DK has a HW debugger on it out of the box (to program the MCU and debug programs), is reasonably priced, works cross platform, and packs a lot of processing and peripheral punch. Being based on the Cortex M33 it has a solid security foundation (TrustZone and TF-M), and works well with a first-class RTOS like Zephyr with open source networking and wireless stacks.

You'll find answers to common questions with this chip on their forums and online.

There are other options -- ST and NXP have many excellent MCUs and inexpensive dev boards -- but the nRF boards and the ecosystem around them make them a good choice to make a serious start and learning embedded, and they are one of the only vendors who reliably answer questions on their support forums. The nRF53 dev board brings a lot of value as a serious learning platform if you're getting started.

Again ... just an opinion!