All those questions are related with iPhone charging IC and Power IC. Face those trouble, you can do some efforts before lose your heart.
(If you have repair device, there are operations you can do. If not, skip the part.)
Step 1: Make sure the voltage of the Battery Connector be 1.5 to 3.7 DC Volts using a Multimeter Pen.
Step 2: Check charger voltage must be 5 to 7 Volts. Check Fuse, Coil and Regulator - one by one. Change if required.
Step 3: If the problem is still not solved, Heat with ATTEN ST-862D hot air gun or Change the Charging IC.
Step 4: Lastly Heat, Reheat or Change the Power IC.
If you have no the mentioned devices, you can operate as the following solutions for not charging issue.
Solution 1: Check both ends of the USB cable you use to charge your iPhone. Apple’s lightning cables are prone to fraying, especially at the end that connects to your iPhone. If you see visible signs of wear, it might be time for a new iPhone charging cable.
Solution 2: Check your USB adapter for signs of damage, like breakage. Don't use damaged accessories. You can try to clean the charging port to remove dust or debris with a compressed air or a dry tissue.
If the issue persists, check iPhone lightning cable and see if it works with other Apple devices.
Solution 3: Clean Your iPhone’s Charging Port. Brush it out, if you find lint, gunk, or other debris in the charging port of your iPhone. You need something such as a toothbrush that won’t conduct an electric charge or damage the electronics in the bottom of your iPhone.
Solution 4: Put Your iPhone Into DFU Mode and Restore. It’s time for a last-ditch effort - the DFU restore. A DFU restore will erase everything on it and restore it to factory settings. There’s a small chance that a DFU restore will bring a non-functional iPhone back to life.
Solution 5: Replace with a new iPhone Battery Replacement.
About iPhone Fast Battery Drain issue
If your face iPhone Fast Battery Drain issue, there are some available methods.
Solution 1: Uninstall the one that is drawing excessive battery and use an alternative app. Put your phone on battery saver mode. Do not use your battery under 10% to extend your battery life.
Solution 2: Sometimes location services also drain your iPhone’s battery. Go to Settings > Privacy > Location Services and turn it off. Use it only when you need it.
Solution 3: True Tone display shorts the battery life. Auto-brightness is the best way to deal with this problem. Swipe your screen upwards from the bottom to pull up the control center and adjust the screen brightness.
If the iPhone battery issue persists, you need check and replace its charging IC or choose a new a iPhone Battery Replacement for you iPhone.