What is FTP, actually?
You'll hear it thrown around a lot. People estimate and measure it differently. Folks use it as a comparison tool but really it's just a number to base your workouts on and to gauge your energy expenditure during a ride.
Functional Threshold Power, roughly the power you can hold for about an hour. Its only real job is to anchor your zones, so what matters is that you get a number, use the same protocol every time, and watch the trend rather than the digit.
Four definitions that disagree
The reason FTP feels slippery is that it is not one thing, it is four or five things that mostly agree with each other and occasionally do not, and which one your app used is usually invisible to you.
The same rider on the same day can come out ten or twenty watts apart depending on which of those you run, which can have a massive impact on your self-worth if you treat the number as your "fitness". The better way to use it is as data for training zones. And depending on how you measure your FTP it could be over- or underestimated. In my personal experience as a runner, I was able to punch out a nice ramp test number that I could not hold for 20 minutes let alone an hour.
What the number is actually for
Zones are defined as percentages of FTP, so the number is a reference point rather than an achievement. Sweet spot is somewhere around 88 to 94%, threshold work sits near 95 to 105%, and VO2max intervals live north of 106%, which means every prescription you follow is downstream of one estimate.
The percentages and the sessions they turn into are written out in watts in cycling workouts by zone, and the workout tool rewrites all of them at whatever number you put in.
That is the part worth remembering. If your FTP is set twenty watts high, your sweet spot rides quietly become threshold rides and your easy days stop being easy, which is the same failure mode as heart rate zones set off a formula rather than off your actual max.
How it became a vanity metric
Somewhere between power meters getting cheap and Zwift putting everyone in the same virtual room, FTP stopped being a training setting and started being a score. Racing categories are drawn on watts per kilogram, leaderboards sort on it, and people kind of size each other up based on it.
I am not above this either. I set a 4.0 W/kg gate for myself before I was allowed to buy a nicer bike. I never really fully got there (3.9 W/kg) and still have a chip on my shoulder because of it. It is the same shape of thing as chasing the VO2max number on your watch.
Getting a number without hating your life
If you've done a 20 to 60 minute quite hard steady effort and have a FIT file for it, you can try estimating your FTP from it - the FTP estimator runs both protocols over the file and shows you how far apart they land on your own data rather than on the ten or twenty watts quoted above. Any old ride will not do it, there has to be a hard effort in there to read. Otherwise you'll want to do one of the tests below.
For Zwift, I prefer the ramp test. The 20-minute test is probably more accurate but the ramp test is a lot less total suffering IMO for a still usable estimate. Just know that the protocol you choose may result in a different number at the same fitness profile, depending on your riding strengths/weaknesses.
Retest every six to eight weeks, or when a workout that used to be hard stops being hard. And pick one power source and stay on that too, since my Kickr Core and my pedals disagree by about 3%, which is enough to make two months of training look like progress or like nothing depending on which one you read that week.