Owner of iPhone 6 64GB, IOS Version: 10.3.3 (14G60). Modem Firmware: 5.62.00

Problem:
-Battery indicator. Example current battery at 80%. After few hour without any usage, % drop to 50% or 60%. The battery seems to drain very fast even with the screen locked. When dropping to 1%, phone shut down. After 5 - 10min, restart back without charging, battery maybe more than 1% (10% or 30%)
-Silent mode turns on/off by itself(sometimes only). DUst issue?

What is the problem actually?
Is it hardware or software usually?
Hard reboot doesn't solve this.
Please advise.
Thanks

Actually, what's the proper way to have a healthy state battery?