(You may need to refer to the General FAQ if you are unfamiliar with Vice City speedruns.)
Q: Why didn't you get that package/stunt?
A: I get it later when it's slightly faster, safer, or doesn't make a difference.
Q: Why are you looking at the store owners?
A: To do the store knockoffs, Tommy doesn't actually point the gun at them using standard controls on the PC version, so you just have to look at them in a certain way at a certain distance with a non-melee weapon in hand and it'll work. Plus, all 15 store robberies are required for 100% completion.
Q: Why do you shoot the store owners?
A: One-shotting them with a hitscan weapon (i.e. any gun that isn't a sniper rifle) prevents them from setting off the alarm and making me get 2 stars. In some instances, it doesn't matter or is even advantageous to get 2 stars though, so I don't shoot them in those instances.
Q: Why aren't you taking the heli on G-Spotlight?
A: Three unique stunts, required for 100%, are done on these buildings.
Q: [SSA Instadeliveries] What the hell is going on?
A: I started Loose Ends with an om0 call, took the phone call and killed a certain number of people in certain locations, then started it a second time (i.e. duped it).
Since this mission has such an absurd amount of NPCs, it starts messing with the game's memory and actually manages to screw up the Sunshine Autos car lists a little. This is abused to instantly complete the lists, but it only does the bottom half of the lists for some reason, so I still collect the cars of the top halves earlier on in the run.
As for why I kill those specific people in the first mission instance, it's because having too many of them makes it so even if I deliver some cars, they sometimes don't even count. It's quite finicky, and you need the perfect balance of NPCs for it to go smoothly.
Also, since it is dependent on NPC spawns, the street pedestrian/gang spawns are mandatory as well. If it is raining or nighttime, it will not go very smoothly either, so I make an effort to set the time to late morning with clear weather before doing this strat.
Q: [Trial by Dirt duping] What the hell are you doing?
A: This involves starting the Trial by Dirt off-road mission many times with rampages, and then starting Vigilante or Taxi Driver.
For Vigilante, starting it after duping (duplicating) Trial by Dirt 12 times instantly completes all 12 levels, with a fairly high risk of the game crashing unfortunately.
For Taxi Driver, the "fare" dropped off in this massively duped version of Taxi Driver will give you as many "taxi fares" as you had running instances of Trial By Dirt before starting Taxi Driver. The destination right off the bat is in fact the one of most recent fare picked up due to a side effect of starting Taxi Driver with Trial By Dirt, so a suitable Downtown destination is set up during Sunshine Autos cars collection, and used to drop off a close fare without picking anyone up. This is split up into two segments of 50 dupes each, because duping it over ~63 times crashes the game. It saves 40-50 minutes over regular duped Taxi Driver and removes a huge RNG element from the run.
Why this works: basically, by playing a saved replay from beyond 2 minutes in "the future", the game will think you've failed a rampage by "running out of time".
This lets you start Trial by Dirt over and over. When you cancel the rampage, Trial by Dirt will restart, but the rampage must be re-activated extremely quickly, otherwise there is the risk of the mission starting on its own without the rampage and preventing the "duping" streak from continuing. This is why the keypresses are the way they are for this, and why there's a 5+ minute waiting period before this: to obtain a replay sufficiently far in the future that the whole segment, with its usage of rampage instant-fails, can be completed.
Here's a video version: