Romantic generally involves a lot of physical contact associated with romance, like holding hands, cuddling, kissing, etc. There are romantic overtures, like going on dates as a couple that often have that contact or at least the potential for it. Basically behaviors that people consider to be things people in relationships tend to do.

Platonic may have some physical contact, but it is more limited and tends to not have the sexual overtones. Friends who are girls may hold hands, but not do the other things like kiss and cuddle. They might go out as a pair, but without any sexual attention. Guys can also hold hands, but in some cultures it is seen as romantic for men but not women and yes that is a double standard.

Honestly it comes down to behaviors commonly seen as romantic or platonic when observed from outside and it is based on intent by the participants. Hugs, they can be friendly, romantic, or cold and done out of obligation and can be intended or seen different ways depending on context. Same with closed mouth kissing.

But that is focused on behaviors. Platonic relationships don’t have the same kind of romantic feelings, which are also context dependent.