How do i use sopo? what are the most threatening things he can do to characters? what are his best moves? how do i make him threatening in some sense?
Sopo is still a very good character once you understand him.
The big thing is movement. I stress this with IC players in general, since most ICs will just kinda chill in one spot and wall and not move much in general, but sopo has even better (or rather, more reliable) movement options than synced ICs
He's still incredibly fast on the ground via wavedashes/wavelands, but as was mentioned, he has a dash dance.
And it's a really good dash dance.
DD to grab/pivot fsmash is really solid against most characters.
Shield drops/shai drops are also way more reliable, which opens up a whole lot since you don't have to worry about leaving nana stranded on a platform or accidentally dropping nana through while you get stuck. This is great because really all the ICs shield drop aerials are awesome in the right situations (fair is pretty bad usually, but it can be used on occasion)
solo-squall and solo-blizzard are way worse in neutral (like you should almost never use them... except in very specific scenarios) which makes dealing with aerial walls, specifically peach/sheik, a lot more difficult to deal with.
solo-squall does still work though, you just have to be very careful with your positioning because it can be punished since it has more endlag and less maneuverability.
combos work exactly the same as normal IC combos (minus desyncs and wobbling/handoffs).
You can still CG a lot of characters with dthrow until around 60% (off the top of my head: spacies, falcon, ganon sheik, pichu, pikachu. There are probably some I'm missing. Probably dk/bowser. maaaybe mewtwo? not sure).
And beyond that, dthrow is still an excellent tech chase option since ICs wavedash will cover so much distance.
You can also reliably dthrow->usmash on most mid-weight characters and cover floatier characters DI/DJ with wavedash usmash at mid percents.
On fast fallers dthrow->dsmash and dthrow->fsmash (at mid-high percent) work well.
bthrow is also very effective as a tech chase option. If you bthrow near the ledge, you can cover most tech options with your wd.
uthrow is a reasonably good combo move. At really low percent, on FFers you can uthrow->usmash or uthrow->dash attack depending on DI.
uthrow->uair works really well on most characters, even though you can't usually follow it up, it puts them in a bad position.
dthrow->nair is one of my favorites, since you can follow it up really well on most characters.
retreating land-cancelled iceblocks onto platforms are pretty nice. I don't iceblock very much other than that since it can be punishable.
sopo's recovery is atrocious, most of the time you'll just keep getting hit offstage until you either die or can reach somewhere (like a platform) where you aren't easily hit.
I can't think of what else is important right now, but feel free to ask more specific questions.
My best advice right now would be to watch a lot of Fly and Wobbles play, since they spend a lot of time wrecking with sopo.
Some other players like Trail and Chu also have really easy to emulate sopos.