i dont think i will be using any of these 10 pieces because i am very sure i have done the SOT223 vias wrongly and the SOT223 will overheat and protest very angrily ! ( all my vias are around it ... but none are under or inside the pad ha ! otherwise, it could perhaps handle very low LM317 loads with the power PNP, there are also other very exciting noob mistakes on this pcb lol)
Generally you shouldn't do vias in the pad itself as they will suck solder into them what could cause serious soldering issues.
Yeah, better to avoid, but used responsibly, it's not as big a deal as it sounds. It's most critical for small pads that don't have any solder to spare. You want to avoid via-in-pad for e.g. gull-wing (SOIC or finer, not that you can get a to-spec via inside anything finer pitch), QFN (side), LGA, etc. BGA aren't as big a deal (there's a whole big ball of solder to spare!), but are preferred to be done with "dogbone" connections. I'd say... it can be done for e.g. larger chip resistors/capacitors, but still best avoided.
It's least critical for large pads with lots of solder to spare (e.g., SOT and DPAK power tabs?), and a non-issue for hand soldering (likely for amateur projects).
QFN (and others') exposed pads are normally designed to use less than 100% paste coverage, so you can kick it up to 100% and add a few vias, anticipating that the vias will wick the excess. I think preferred is to use solid or plugged types, but these are more expensive, and unplugged vias are second best. (Tented vias are least preferred, because tenting inevitably traps gas that causes solder voiding.)
Very thin vias (generally 12 mil and below) are thin enough that 1. they don't soak up much anyway, and 2. they don't often soak to begin with -- especially on RoHS processes, it seems, solder doesn't flow into small vias. This is another reason small diameter vias are preferred around fine pitch stuff.
Tim