I have learned the most about this game by watching Let’s Play videos on YouTube.
I highly recommend the following two series to watch…
ENDLESS SPACE 2 Let’s Play - VAULTERS (New Expansion) - #1 (Macsen Lets Plays)
Vaulters - Let’s Play - Endless difficulty - Endless Space 2 - #1 (FMRPlays)
Even if you’re not interested in playing the Vaulter faction you will learn a great deal about the game basics watching these.
As mention earlier in this thread by AquaMafia, FMRPlays has a great series of tutorials for the game that I found very helpful as well… Endless Space 2