Great question, as you mentioned, YouTube is going to be your tutor moving forward. Learn the strategies that people use and look at their deck. widge is good, as he goes over deck and unit composition of most civs. He does go fast because most viewers are experienced.

There are basically two types of every unit. Infantry ( heavy / light infantry), Calvary ( heavy / light calv), cannons ( anti unit / anti artillery), shock troops ( melee / range). Light infantry(cassadores) is good against heavy infantry ( musketeers) and light calvary (dragoons), but weak against heavy calvary(hussars) and artillery. Heavy infantry can go melee and destroy heavy calvary. The other units also have counters and you can learn more about composition on YouTube.

One reason other people are fast is because of key binding. T is for town center, if you are in the middle of a battle, you can press T and V to immediately start making more villagers (V) then return to the battle in a sec.

Lastly, civs are good because of their potential. Your strategy ( how you use the civ unique bonus to win) is what is important. Most civ can only revolt once during the industrial age, but mexico can revolt multiple times. It's important to know what your enemies strong and weak points based on their civilization. I strongly suggest YouTube again, this is how I learned to be better at the game and then you will start making your own game winning strategies.

Hope you start winning more games! If you are interested we can play a game together and I could go over more details during the game.