Few beginner questions - from AVR to ARM

hi,

after some time i think its time to learn arm programming, i used to program for avr, (im mainly a pc programmer ! young one )

i got an Raspberry pi model b+. but i don't want to use it for making fun stuff or hobby, i want to do real programming(not that python and pi are not real programming i can't explain what i mean) , so i can work somewhere(i don't want to but example of what i mean, im not looking to make stuff, im looking to learn!!!) or make products! (like nest self learning thermostat or the Parrot ar.drone)

raspberry pi has an os and its a big boards and has unneeded stuff so i don't want to use it on my final product (release it with a pi inside of it!)

like in avr i wrote my c program for my chip on breadboard, or an arduino, after that i just installed the final version into  a chip ans inserted it into a final cricut , but not the whole arduino! (like lets say i made an automatic cat feeder and released it for public!!! )

i want to do the same, so my question is:

how can i start it? Do i need to know Assembly?  i only know a little bit of PC assembly, it sounds cool

i heard different MPU/MCU's require learning the whole thing again, i may use different stuff on different projects, that means i have to learn it again for each project?!

lets say my goal is to make AR.Drone from scratch, by buying motors the chips and ect, but i want to prototype it with pi first, and the OS and other stuff just require battery and waste resources , how can i do it?!

so what should i do/learn to make an AR.Drone?! (i know its not like i wake up and decide to build a time travel machine but i will learn what's necessary because i have time and im looking to learn !)

sorry if this question sounds stupid but i have been really confused these days so i thought its good to make a thread!

EDIT:

let me summirize it a little !

Summary:

im moving from AVR programming to ARM,

what steps i should take? what should i learn?

i want to use ARM for making products and devices , i already bought an pi to prototype before i can make it a product , in its own board with other stuff in the board

a good example of what i want is ar.drone !