I was given Civ 6 as my birthday present last November, and I've put in many hours on it since then. Currently, I'm only playing at the Deity level (which is the hardest).
Here's my thoughts about how to do well at this.
First, choose a good civilization. Germany is my favourite, because the Hansa (industrial district) is much cheaper, and you get u-boats (other civs only get submarines). And an extra military policy slot.
A six-armed snowflake or 4-armed star is good, because the give you a chance to develop your civ before you get attacked. Because you *will* get attacked; at this level, the other civs are very aggressive.
The way to deal with this, is to have a strong military; that deters attacks, because if you do get attacked, you're very likely to lose. But, of course, expenditure on military reduces all other expenditure. It's a balancing act.
Get crossbows as soon as you can, and then go for field artillery. Put a wall around the city that's nearest to the other civs, because for the first part of the game, you're going to be entirely on the defensive. But you will want to colonise as much territory as you can defend, because you'll need the aluminium resource later.
After getting field artillery, you want to go for electricity, because that lets you build power stations, which means the best production capacity. You also want campuses, to speed your research.
The next target is to build up your treasury via trade, and building commercial buildings.
Another good way is by selling stuff to other civs. You can use trial and error to find out the very most they're willing to pay for your luxuries (or strategic goods). And there's what looks like a bug, whereby the other civ will pay you what they promised double or even treble. I feel sure that the programming team must know about this by now, so I expect that will be fixed. Meanwhile, you're defending (using field artillery), but you will want a few knights, which will be used in your aggressive war later.
After electricity, go for advanced flight (that's why you need aluminium), because that gives you bombers. As soon as you have bombers, use the cash you've accumulated to buy as many of those as you can afford; at least four and maybe eight. And with this, you can go on the offensive. Bomb the target city until it's dead, then send in a knight to occupy it.
Artillery is pretty useless; you'll only want it if you haven't got aluminium to make bombers, and even then you'll only want artillery until you can make bombers. The problem with artillery is the short range (two hexes, or three if you use a balloon spotter) and slow movement (two hexes). Bombers have a range of ten hexes, and you can move them from anywhere to anywhere (another bug, I think - you're probably only supposed to move them 20 hexes).
With only one aluminium resource, you can make as many bombers as you want. I suspect that's another bug - if it is a bug, it's a very important one that really makes the game very different from what I suspect was the intention.
First attack your weakest opponent. If they offer to surrender most of their cities (i.e., all except their capital), accept that, and make peace. Then attack another weak opponent - you can come back to the first civ and polish them off later.
Religion is irrelevant; I can't see how you can ever win using that. Culture doesn't do too much either; your best bet is World Domination, meaning "wipe out everyone else".
Once your offensive is launched, stay on the defensive against the other civs, and direct your research towards Stealth, because that gives your bomber fleet a big boost in power and range. And attack the other civs one by one, until you achieve --- World Domination!