fly leader/tippet

Floating line when overcast, sinking when direct sunlight is on the water.
A leader can have a dry at top, bottom, top and bottom, or all three, with wet flies in the other places.
Most people use the dry on the top dropper and slimline wet flied below, with the key pattern on the bottom. This is called traditional style.
Leader turnover is easy if you place a weighted fly or an unweighted fly with a "water soak" body, which will be heavier after it's wet.
With such fly selection no taper is required, but it's bright to put a wear and tear stronger section to the flyline, and tie your leaders (tapered or not) to that.

If you get tangles with droppers, gradually reduce the fly count until the tangles go away. If that means no fly is left, more casting practise is required! :-)

I never took to that x system.
It goes back to the days when real animal gut was drawn (pulled) through metal dies with tiny holes and cutting edges, to shave the gut down to a certain thickness.
The more dies it got pulled through, the thinner it got, and the higher the x number.

Modern lines are measured in diameter, by mm.
But even diameter is not a good measurement, since the line may be coloured dark, or light and light up, becoming more visible than it's diameter would otherwise suggest, if fished in bright daylight. Even clear fluorocarbon lights up in sunlight, though the makers would have you believe otherwise.

Mono in 3lb is quite shock resistant, or fluoro clear in 5lb is less shock resistant, and has bad knot strength (relative to mono), but might be as good as 4lb mono. Either are good for fussy fish.

the deeper you fish the less visibility matters. I use 6 to 8lb mono off fast sinking lines, 4lb mono or 5lb fluoro off slow sink and intermediates, and for floating lines I guess 3lb to 4lb mono, and 5lb fluoro are what I use mostly.

But if during fishing I got a feeling of unease with the trout's responses, I'd change down in an instant to check that. It's quite eductaional how after a fishless 20 minutes and then changing diameter down, the very next cast hooks a fish.

The fluoro is heavier and consequently sinks faster, making it superior to mono when nymphing is planned.
Mono is stretchier (only by a bit, the fluoro makers are economical with the truth about stretch) and it's higher knot strength wins out when going very light.

When making a tapered leader (with knots!) I would go (from the 12lb collar on the end of the flyline) 10lb, 6lb, tippet. But that might be 1 day in a whole season, and for a 20' leader.
The rest of the time, I work on the casting and use a level leader from 9' up to 15'. My knots are smooth and go through rod rings ok when bringing a fish to the net.

I didn't mean to confuse, but it's difficult to explain without writing a book ....

Turning the leader over is commonly accepted to be done by the flyline, passing it's motion into a stiff leader, OK?
So the idea is that a stiff leader butt end, brings more motion down towards the fly and makes it turn over and all land nicely in a line.

Now here is the problem with that ... a stiff leader is often a thick leader, and you're putting that near the fly where it can be seen.
Also ... the stiff leader affects the fly on a moving water as in a stream more than a thin supple (easily tangled!) leader would. And the fly should be free as much as possible.

So here is the problem: a good leader for turning over and casting well is simultaneously a bad leader for sneaky invisible presentation as soon as the cast finishes.

See what I'm getting at?

Now everybody has their own compromise, based on balancing their personal casting needs and ability, against their success rate at fooling the trout. That's why we are all different in this.

But there is another way, so long as you are fishing flies "wet" you can use weighted flies to contain momentum: at the end of a level leader, instead of using the upper stiff leader section to bring the motion down to the fly.
Multiple flies also helps do this.
So when you get to the stage where dropper-to-leader tangles happen less frequently (casting technique) you can fish more flies and use non-tapered leaders.
Until you can do that you have two choices: either a tapered leader or a weighted fly on the end.

Dry flies and bulky flies that penetrate the air badly limit the alternatives but a beadhead nymph on the point and a dry buzzer dropper like a grey duster still works very well off an untapered leader. Just watch the knot strength if using fluoro, and test for wear frequently.

They all work, but they don't all suit everyone, and changing a wet fly to a dry fly can make a lot of difference to which leader suits you better.
So I recommend to try it both ways and make up your mind. But if your casting gets faster or neater later on, test both again, your preferred leader will change too.