To be honest, that is how the original civilization games feel like. Very sparse and unfriendly onboarding.

Civ2 at least had a tutorial game (explained in the paper manual!) and auto "help" ("this looks like a good place to build a city"). The game is pretty hard until you get the hang of it, especially in the version where the AI hates humans.

My cousins always want to play Civ III, and after several ~10hr games, I still don't really know what I am doing.

It's a game of decision-making. Each decision leads you on another branch of the tree. To make it more unpredictable, your decision-tree is mutated by each other player's decision-tree.

Like any traversal-oriented game (RPG, etc.), it just takes a lot to understand the effects of your choices, and what goals to make.

After a few games you kind of get the hang of the initial phase, and some techs are always decent to aim for at first, but the really tricky part is quickly assessing your starting conditions and​ adapting your goals from that.

I suggest watching some tutorials on YouTube: https://www.youtube.com/watch?v=K6xBi2JWyZU

I don't know about Freeciv, but all other civ games have extensive guidance systems. Game tells you "build your city here", press B to build city, build defensive unit, fortify unit, go explore the terrain, pick this research, build this building". Sure, if you're an advanced player, you can ignore that and go your own way, but as a novice player you can just follow the AI advice and play on a somewhat comfortable level.

Civ1 was the same, you gotta know the basic hot keys A, S, X if you got them down things get a bit easier.

That reminds me of when I got Civilization I for my birthday.