It is playing Civ VI with the hardest game modes introduced with the New Frontier Pass: Zombie Defence, Apocalypse (and Dramatic Ages thrown in for extra fun!)

While there are guides for how to play the individual modes: How To Survive Zombie Defence, How To Survive Apocalypse Mode (and How To Play Dramatic Ages), which are essential prerequisites for this guide.

There are some not obvious(?) synergies playing the two (or three) modes together.

Welcome to my zombie deathtrap.

Map: it is essential you pick an arboreal map. That is one with lots of forest. Or rain-forest. Firaxis' Europe map is a very good one to start off with.
Civ: Vietnam, Bull Moose Teddy Roosevelt, Kupe, Varangian Hardrada, Unifier Qin, Gitarja, Ludwig, Brazil, Wilfrid Laurier, Kristina, Nzinga Mbande are good suggestions.

• Exploit Hyper-yields of Apocalypse mode to super power city production, enabling quick turnarounds for Turn Undead and Dark Signal.
  
• Use Soothsayers to set fire to forests (or rain-forests) to burn zombies.
  
• Build cities on islands, coasts and locations hidden by forests/rain-forests and mountains.
  
• Beeline Conservation to plant forests. (Or Medieval Fares as Vietnam).
  
• (Optional) Build a few Industrial Zones for Carbon Recapture (if you haven't already won).

(Scrambled South America)

Counter to standard Civ VI strategy of early expansion (or rushing), you focus instead on:

• Exploration: Build a couple of explorers to scout out nearest neighbours.
  
• War: Capture workers. (And settlers). From your nearest neighbours. You are warring to protect the swathes of forest/rain-forests (that will ultimately hyper-yield).
  
• No-chop! It is essential to not chop your precious forests (or rain forests) playing Zombie Apocalypse Civ VI.
  
• Build: It is essential you beeline and build either a Holy Site or a Campus in your capital.
  
• (Optional) Settlement: Only islands. (E.g: Kupe or Varangian Hardrada).

The primary reason you don't bother expanding in the first era playing Zombie Apocalypse Civ VI is the zombies will kill your cities. (This is why Dramatic Ages plays well in Zombie Apocalypse).

Don't fret if you are in a dark age for the first era.

• Expand: If you haven't already, start to settle islands after researching the tech allowing embarkation.
  
• Build: Improve resources, especially strategics and luxuries. (You may harvest bonus resources - useful if you are not chopping).
  
• Defences: Start to build walls and the Zombie Defence mode improvements, trap and barricades.

• Defences: Continue to build defensive improvements. Notably the reinforced barricades and modernised traps.