I don't work with webtoons or really post on there, but I do read a lot of webtoons, so I kinda know what is and isn't allowed.
Curse words are normally fine, but I think they prefer it if you censor the word a bit. Nudity is normally fine as well as long as it isn't too sexual. Shirtless scenes are totally fine, especially if the character is male (I'm not sure how they feel about female nipples though, so be careful with that). Blood and violence is also allowed, but comics that often have stuff like that are marked as mature and the reader has to confirm that they are of a certain age before reading (for example Uriah). Discussing sexual topics should also be fine.
For the brand censoring, thats because of copyrights and trademarks. Comics normally censor out or change a brand/company/song/artist/movie/etc name because they don't have the legal rights to use it in a webtoon that they are being paid to make. Most people don't know which brand names and stuff you are and aren't allowed to use in commercial material, so they normally just censor/change any names they think there might be an issue with.
If you were to get featured on webtoon, you would get an editor who would know all the rules that your comic has to follow. So you don't need to stress too much about it. As long as your comic isn't 18+ (unnecessary sexual content, lots of unnecessary violence etc), it should be fine.