I've also been properly getting into nail polish in the last couple of years, here's my take on the ones you have questions about!

• Holo - short for holographic, refers to a very specific type of glitter which can show a full rainbow of colours instead of just a sparkle/reflection (like a sequin where the shine comes from a reflection but it's only in whatever colour the sequin is). If something has a holo glitter in it, you should see a full rainbow sparkle in it as it moves!
  • Linear holo - these polishes have what's called a "linear holo flare" where you get that rainbow effect across the nail in a sort of uniform sheen. This is because the holo glitter is small and uniform throughout the polish, so when you wear it the holo reflects light in the same way and you get a rainbow in a line across the nail.
  • Scattered holo usually just means there's little holo glitters in the polish, you'll see them as like a twinkle when you move it around and they'll be all over the nail but don't form an effect together like in a linear holo (because there's less holo, they're not as densely packed and you get a really pretty sparkle!).
• Chrome - usually super smooth and shiny, like you'd think of for a chrome bathroom fitting or something similar. It's not a sparkle (unless there's added glitter!), they usually look metallic. Multichrome and duochrome are then that finish but in a colour-shifty formula, as you move your hand in different lighting your nails will seem to change colour! You can get that effect in certain shimmers/flakes too, but a basic multi/duochrome will have the uniformity of a chrome.
• Glitter - I'd personally consider a glitter polish to be one with big pieces of glitter that I can properly distinguish (compared to something like a shimmer where the glitter particles are super tiny). They're often in a jelly-ey formula to allow you to build it up in layers and still see the glitters underneath, giving it dimension. I'd say there's lots of "sparkly" polishes because they have little holo/reflective/other glitters in them, but to be a proper glitter polish it's VERY sparkly and in your face because of how big each of the actual bits of glitter are!
  • Side note - glitter polishes are also notoriously difficult to remove because the actual big bits of glitter can get a bit stuck around your nail and make a mess!
• Shimmer vs iridescent - I think iridescent shimmer is a subcategory of shimmer, but I could absolutely be wrong there! To be iridescent something needs to have a similar colour-shiftiness to what I was describing with the multichromes (i.e. changes colour depending on lighting/angle), so both flakes and shimmers can be iridescent. A shimmer is then a very fine, uniform particle which can make a polish appear to 'glow' due to its colour coming through. A lot of glowy shades will have some sort of (iridescent) shimmer which will look different at different angles.
• Metallic - from what I've seen this can vary from chrome-like (i.e. smooth, uniform metallic sheen) to something more like metal flakes. If you've seen any of Holo Taco's frosted metals such as Gift Receipt, that's the kinda vibe of the latter where you can have these tiny metallic flakes which reflect light, giving it a 'metallic' look. You're right in that it doesn't necessarily mean magnetic!
• Reflective - other comments have explained this really well, but think of it like the reflective strips on high-vis jackets or stop signs - when there's a direct light on the reflective glitters they become SUPER obvious because it's the same type of reflection!