So in Civ 7, leaders and civs are separated. At the start of the game, you’ll pick a leader who you play as throughout your run, as well as a civ.
The civs available to you depend on the age, so if you start in Antiquity you’ll have appropriate civs from that time period to choose from. When you get to Exploration, you’ll get to choose another, and so on.
You’ll keep everything, it’s just that your name (I think) will change, and obviously, each civ will have its own strengths and weaknesses, different units, art, etc. I hope that, since they’re now tied to their age, it’ll allow them to really focus on what made them unique within their time period, rather than having to make the Roman Empire make sense in the Modern age.
I’m not sure what Civ 7 specific features there are, but I’m sure their options will depend on your chosen civs as well.