Answer 1

Try using different threads for different questions
https://www.sololearn.com/discuss/333866/?ref=app
Now your answer
I would recommend you to visit these codes(by 🅰🅹 - ɪ'ᴍ ᴄʀɪᴍɪɴᴀʟʟʏ ɢᴏᴏᴅ! )and run them. They will answer most of your questions

https://code.sololearn.com/W56O4ut2QeLD/?ref=app
https://code.sololearn.com/W5vTxD5t5TKh/?ref=app
https://code.sololearn.com/W049XB43HrqG/?ref=app

Answer 2

For android use Android studio & Java, Kotlin or C# (Xamarin framework).

For creating databases use MongoDB, firebird or sql (MySQL)