Start with Arduino/ESP32, tons of great tutorials. Learn how i2c works, kirchoff's circuit law, and ohm's law, and play around in the Falstad simulator that runs in a browser (https://www.falstad.com/circuit/)

Then find an appropriate sensor module breakout board and connect things up.

Other that the very basics, what you should focus on highly depends on whether you're a "Mad science analog tinkerer" or "Ultra modern high tech" kind of person.

If you're serious about this stuff you probably want to learn 3D printing to make cases.

I think the most important thing is reasonable expectations. Commercial products take months to design and large teams to handle all the companion apps and stuff. Either keep your project very simple, use off the shelf stuff wherever possible, or be prepared to spend a lot of time on it.

And be aware of the time and effort required in physically assembling things.

Despite having done this stuff for years, I still don't actually have any custom hardware in use at home, because commercial stuff is 80% good enough, and has warrantee support and all kinds of features that would be a bit time consuming to DIY.