Hello fellow pianists! New member here. 
I can't say I'm an advance student. I'm 15...
I usually spend 6-9 hours in practicing starting with scales and arpeggios from Hanon. I took the preface seriously believing this will improve my technicality. But I've been seeing a lot of posts saying "Scales and arpeggios will not improve piano technicality.", "Etudes are better.", "Don't spend too much time time in practicing...", something like that.
Well, er, I really hope you don't spend 6-9 A DAY practicing. That is too much. First, it won't do you very good. Second, I really cannot understand how you manage to put so much time in practicing when you are 15. You have school. You need physical exercise, simple leisure time, to socialize with people and - YES!!! - help your family with household and everyday tasks. All of these things are just as important as piano playing ... even if you want to become a professional piano player.
1. How to the make most of your piano practice?
2. How to improve technicality, interpretation, memorization, sight reading skill, etc.?
3. What are your tips and suggestions in practicing?
4. What's your daily piano routine?
5. What do you think are the daily habits that can make you a good pianist?
6. What are some piano works/exercises that can help you?
7. How did Beethoven, Chopin, Liszt or any other piano virtuosos practice?
8. What are your personal experience/s that seems to improve you?
9. What tools/apps do you use that helps you in your piano education?
I just described in another thread that my teacher last week ordered me to restrict my playing to 4x15 minutes a day, with at least 10 minutes physical activity inbetween sessions. This is to make my learning curve even more effective. Practicing and learning is not just a conscious, active process; there is a lot of subconscious processing as well. In the future I will play more again, of course, but this exercise is doing me very good right now!
It is also a well-known fact that your ability to learn significantly decreases after 15 minutes, so taking a break there is always a good thing.
I do not play Hanon and similar exercises. My teacher has given me a sheet of paper with some exercises, but most I simply invent exercises while I work with a piece. If the piece in question is in c minor, I do some technical exercises in c minor, for example. There are always scales and arpeggios in classical pieces, at least most of them, so if I see a scale passage, I practice that section. I might experiment by doing it with both hands, or the other hands, or reverse the passage, or extend it over more octaves, or trying it punctuated or ... whatever.
I like the e-books and online tips by Graham Fitch, check them out. One advice from him, borrowed from Liszt, is: "think ten times, play once". That is, don't feel forced to produce sounds all the time. It is better to plan and think a few times more.
Also use the "feedback loop": plan what you want to achieve with a certain section, then play, then evaluate carefully. If things went wrong - you still felt tense, you still made that mistake, you still felt you got the wrong expression - you must think of how to correct it and then you do it all over again.
When you feel you are getting a good hang of a piece, you should spend MORE time - not less - practicing very slowly. This will stabilize your playing. This is especially important right before a recital. Your last rehearsal should be in very slow tempo!
Record yourself often and listen a lot to yourself. You will hear a lot of things you were not aware of while you were playing ... unfortunately.
If you play from memory, can you stop playing in the middle of a section (or at the signal from your teacher), put your hands on your lap for a few seconds and then keep on playing? Can you do that with just your left hand then? And then, at a given signal, switch to your right hand only? If you can do these things, you have certainly got the piece well memorized. Otherwise ... you should work a bit more.
(Personally I am terrible memorizer, I cling to my notes as a baby to her teddy bear, but I'm working hard to improve.)
I use a simple app called Compound Time, which simply helps me to keep track of my sessions. I get a kick when I see the records of how much I've worked. It got a bit boring after this 4x15 assignment, though. And there I have the material from Fitch, as mentioned, and I have also read the book from Chang ... Chung ... I can never remember his name ... But it was good, and gave some very interesting perspectives. I learned "the old school way" 30 years ago, and even piano education has improved since then ...
Answer to question 5: See above, about what other things you should do except sitting at the piano. All of them will make you a better pianist, and a more successful pianist. Otherwise I also think it is a good habit to plan your practicing carefully, to train your ability to focus (rather than your endurance with marathon practicing sessions!)
... and, finally: use your energy on having fun at the piano, enjoying the music, embracing the music and the great works. Don't spill a second, though, on comparing yourself with other young pianists. I am at this grade and I spend so-and-so much time on practicing and I have been in this competition and blah blah blah. The sad truth is that world does not give a sh*t. There are millions and millions of good pianists around the world, you cannot compete by comparing yourself to them - the only way to succeed is by being uniquely YOU.