I saw a French title and almost had a heart attack !
To answer quickly :
Royal Road : More visited, you will have more readers, but the competition is harsh for the novels in the same category, and some authors don't hesitate to review-bomb their competitors. I would say it's high risk/high reward. If you play the popyularity game, you can climb up fast. The genres that are more popular are general adventure, fantasy, gate/tower emergence, litRPG
ScribbleHub : It's the more 'relaxed' version of Royal Road. Less people, less conflict, less competition, if you are a hobbyist it's better to be here (but you can also post your story on both websites, there is no exclusivity clause). More popular genres : LitRPG, smut, genderbender
Webnovel : This is a website that can allow you to directly make money since you can monetize your chapters. You also have the option to be directly signed with the platform, gaining a steady income as long as you can maintain your rythm (if you have to post a chapter every 3 days, you mustn't be late or the deal is off). It isn't a bad website if your goal is to have a job, it depends on your views
Wattpad : Everything can go on Wattpad. You have people writing LitRPG webnovels, others writing One Direction fanfics, and in the middle you have 10 year olds presenting their first short story. Since it's for all public, the app is swarming in stories and you can be burried easily, but it's also the most 'all public' platform, and you can reach casual readers that wouldn't know about webnovels otherwise