Ya you probably shouldn't do long cardio workouts to gain VO2 Max fitness. I doubt long cardio would produce VO2 Max results for most with a good base fitness. Intervals are the time-crunched athlete's cheat to getting results before or after work. Some ppl don't like them because they can be monotonous and they are punishing. But they work.
VO2 Max is the effort zone that you can do for 10-30min, harder than Threshold at ~60min and easier than Sprints. VO2Max probably translates to a HR just above 90% of max. Maybe that's a 3K-5K race pace for runners. To gain fitness there, you need to spend time in that zone with some amount of rest. My intervals tend to add up in duration to the ranges I just listed. 2min x6 per set, x2 sets = 24min. The 1min rest between intervals makes it all possible, and I do 10min recovery between sets. If the interval was longer, I'd increase the 1min rest some too.
Anyone with a modern Garmin watch or cycling computer should have "Daily Suggested Workouts" that include various intervals. Those have added a lot of variety to my intervals and it's cool to know that Garmin thinks I should be doing XYZ. If you don't have such a device, I'm sure there are plenty of online calculators that will take max HR, running pace, and/or other data to spit out some useful intervals.
FTP is Functional Threshold Power, ie the power one can hold for an hour. Cycling is borderline obsessed with this number. Riders can take tests to get it and most cycling computers will give you one after recording a bunch of rides. Mine is set at 247w right now. I use that number to calculate the required power for all my intervals. Well, Garmin let's me input percentages for workouts. So when my FTP changes, the workout changes too.