Suggestions for streaming

Ok so I'm in no way a streamer, but I do am a viewer! So, while not all, I can answer some of your questions.
1)-2)Either Twitch or Youtube. Both automatically save your streams, and I think you can reupload a Twitch stream as a YT video (or at least you could). I have no idea how streaming on Facebook would work, so I can't say anything about it. While being a Steam fanboy, streaming on it... it's not the best. I'd avoid it, due to streams being laggy or not letting people in, chat bugging... I've seen it work, but I always personally had a bad experience with it. Also Steam doesn't allow you to save your streams (AFAIK).
3)I know it's what everyone says, but play something YOU like. IMO, it's the most important thing. Because you can feel when a streamer is playing just to stream the game instead of having fun with it. I'd avoid maybe games that require your attention for long periods of time (EG: FPS games), since you won't really be able to look at the chat... Oh yeah the chat. I'm kinda going off-topic, but mind the chat. Like, you're the star here, you're the one streaming, but people super-like it when you read their messages and answer to their questions. Of course, it depends on what kind of streams you wanna do, but, like, saying "hi" back to who says it to you always makes the difference.
4)I believe they are (or at least were) called Twitch Extensions. If you're planning to do "non-gaming" streams too (EG: just chatting, karaoke, drawing, reviwes, basically everything but gaming) they're a fun addition. And then there's the one that plays funny sound effects when you type in the command. That one is ALWAYS funny (to my smooth brain, at least!). I think it is (or was) called Sound Alerts.
5)No camera is fine. Just talk constantly/regularly, so people know you're there. Even if no one is watching, because if someone were to join and didn't saw nor ear you, it would be almost like you weren't there. So yeah, talk a lot! (There's also that new V-Tuber avatar thingy but I have no idea what it is or how it works. Sorry)

I don't know how useful I was, if I even was at all... But I hope you got some of the answers you were looking for. And be sure to let me know when you start streaming, I'd love to check you out! (Except if you stream at, like, 3AM. My beauty sleep comes first, sorry).
Good luck and, most importantly, have fun!

Think I can help answering your questions :)

1. I'd always prefer Twitch over youtube, but youtube gaming grown rapidly in last few years so it maybe worth streaming on both if you can handle it.

2. By default on twitch your streams are saved for 30 days until you decide to keep them or not.

3. There are few factors here, first depends on what kind of audience you want to build up, if you play competative games play them ONLY if you are good at it, viewers want to watch skill not sucking at it. Most important do not stick at one or two games, then your viewers will be only people who play that game and channel won't grow. NEVER play brand new AAA or hot games that everyone streams, your chance of getting new viewers is close to 0, unless you get Early access to a game that has not been released. Also the game you stream must be fun to you, if you don't enjoy it nor the viewers will.

4. Extra software and extra hardware! Firstly 2nd monitor is a must, or you will have a lot of trouble interacting with chat, also important is good mic, be ready to spend at least 100$ for beginner mic (Blue Snowball, Blue Yeti or Audio-Technica AT2020.) Optional I'd suggesting getting a stream deck that will bring up your stream quality and fun on next level.
   As for softwares all you need to start is Streamlabs OBS it's easy to understand and they have everything you need. Optional you said you are shy for camera, so maybe FaceRig is the thing for you.

Some more tips: Always talk talk talk, practice in front of the mirror talking to yourself, because that you will do when there are no viewers on stream. First 10 seconds are most important when someone joins your channel, and if they see you not talking gets impression of boring and they will leave. I can't explain how much talking is important :)
Streaming time,, stream as often as you can best would be 5 or even 7 days a week with more hours the better. And please always have in mind that you should do this for fun not the money or the benefits of it. It even may take a year before your channel start paying of maybe not even then it all depends on you :)

If you have any specific questions feel free to add me and we can talk, I've been streaming for few years and work on few channels as moderator :)