When an idea comes to me, I jot the general idea in my notes and then slowly build upon it. If I continue to come back to it and keep adding more ideas and there doesn’t appear to be any holes in the plot, I strongly consider turning it into a story. At which point I will then make general outlines for each episode (I only plan a couple episodes out, because my ideas change so much as I’m writing. I do my best in this area).
The type of story I’m writing, mostly. I’m very imaginative so usually I can just close my eyes and imagine how they are, look wise. I also try to think of how their past has affected them to determine their appearance and traits.
I don’t usually plan this, I just do it if it feels right or I KNOW from the start that a particular person has a purpose. But you should make sure that each person you want to add, develop and focus on has a real purpose.
Example
In one of my stories, I didn’t plan on one man to appear and have a significant impact on the entirety of the story. When I did introduce him, I built his role as I worked on each episode, to where he became a completely changed and developed person by the end. It turned out so much better than I imagined because he was like a spur of the moment role. Sometimes spur of the moment ideas are the best ideas 
This is a hard question. To me, it just depends on your story and how all you want involved. But don’t add more characters than you can work with. One of my stories has 2 main characters and a couple dozen side characters. In my revamped version of the story, I’m trying to work on bettering their personalities and development, but it is very overwhelming having so many people to work with, ya know lol
As for developing them all, I think you just need to take their role, if they affect the ending and how often they’re present into consideration.
I try to strictly follow the “show, don’t tell” motto. I’m not very good at writing elaborately in the first place, so I avoid narrator dialogue at all costs. I stick to mostly dialogue between characters, and a very visual story telling experience (lots of animations, tons of overlays, custom poses etc).
Lol I just pick backgrounds I like and that make sense. I don’t usually reference real places and avoid any serious traveling.
Pros and flaws. I usually tend to have more flawed MC’s so I can show their development over the course of the story.
Their backstory. You’re essentially creating a new person, how old are they? Where did they come from? Their favorite color? Etc. You don’t need to spew all this information the second the reader meets them, rather it can slowly be shared sporadically and at the right times in conversation.
Uhmm maybe a little mix of both? I get annoyed when it’s narrator bubble after narrator bubble talking about a character. Mixing the description in with dialogue helps paint the picture of their looks, and then you can spread that information out instead of dumping it all into 5 straight minutes of character description 
I avoid writing about things that I would have to do extensive research on. I like to write purely for fun and typically my ideas don’t include anything I’d really need to do research on anyways.
I do think , however, that if you plan on using characters form other cultures and holidays from other cultures, that it should be researched and have a thorough understanding before using it. This also applies to using foreign language slang, pet names and special clothing/accessories. If you’re unsure, ask! I also think when making foreign characters that you should make sure they’re represented physically as accurately as possible, unless there’s a reason for why they appear the way they do.
Example
In one of my stories, I created her and her family to be Native American/African American. She has features from both sides of the family represented as accurately as I could. She also has blue eyes, which is explained in the story through dialogue with her mom that she got that rare gene from her European grandmother (moms mom) which strengthened their bond since mom loved grandmother very much but she passed early on. I made sure to explain this because lots of people like to create dark skinned characters and give them “white” features such as blue eyes, small noses and small lips with no explanation. My stories been published for a few years now and I’ve never gotten any comments on her look so I’m assuming my explanation is satisfying, makes sense and no one feels offended by it.