a) How bitcoin transactions work:

https://www.youtube.com/watch?v=Lx9zgZCMqXE

b) How Lightning transactions work:

https://www.youtube.com/watch?v=rrr_zPmEiME

c) The Long Road to SegWit

https://bitcoinmagazine.com/articles/long-road-segwit-how-bitcoins-biggest-protocol-upgrade-became-reality