Could be a bad battery or a loose connection. I’d open the phone up and check the battery connections first. If that doesn’t work, try another battery.

Technically the glass only is replaceable. But the shop you go to may not do glass only repairs for liability reasons. Without the proper equipment, it is incredibly easy to damage the underlying OLED screen when attempting a glass only repair. They may also refuse due to the screen burn-in. They may not want a phone with a “replacement” screen leaving the shop in that condition. (The screen burn-in). It can look bad on the brand. Tl:dr You can have the glass only replaced, but it’s hard to do and many places don’t do it.

Put the old battery back in and see if it does the same thing. It could be a bad battery. If it still loops with the old battery, it’s likely unrelated to the battery. It could also be loop disease. The phone will run a self-diagnostic on startup and if it detects something not working (such as the audio IC) it can cause the phone to take a long time to start up or not start at all.

Honestly you are probably better off getting a new phone at this point :-/ But if you are wanting to fix it, if it’s the power button assembly cable that is broken, it can be replaced. You just have to basically take everything out of the phone to get to it though. If the connector on the logic board is the damaged piece, it requires microsoldering to replace and will way outcost the value of the phone.

The 5 series has an issue with the U2 IC (charge control microchip). They can often be damaged with aftermarket charge cables. If you have a ammeter, such as the portapow, you can see if the phone is actually pulling a charge or not. The issue could also be the battery itself. If it is worn out too much, it won’t hold a charge and could be causing the issue as well

There are multiple microphones in the phone. The microphone that is used during a normal phone call is at the bottom of the phone. Video, Siri, voice to text, etc tend to use the microphone located at the top of the phone next to the earpiece speaker. I would double check and make sure the microphones attached to the charge port are lined up properly and that the mesh openings at the bottom of the phone are not clogged with dust, dirt, etc. If both of those things check out, I would try a new charge port assembly as the microphone itself could be bad or damaged.

The easiest thing to do to rule out any software issues is to do a full reset of the phone back to factory. (Backup the phone to iTunes first). If the problem persists, it’s likely hardware related. I would try a new screen in that case. The iPhone 6 Plus has a history of touch disease but since you have your phone listed as a 6s plus, that shouldn’t be an issue. If a factory reset and a new screen don’t fix the issue, it has to be something deeper within the phones logic board.

Did this happen on its own or after a screen replacement? Have you tried doing a reset of the phone? If it happened after a screen replacement, I would try replacing the home button extension cable that is attached to the thermal plate. It can get damaged during the repair and cause the home button to malfunction.

On startup of both the 7 and 8 series iPhones, the phone will run a self diagnostic to be sure all hardware is working correctly. Normally the front camera assembly and home button are the easiest to damage during the repair. Since you have already tried a new lcd and front camera, I would imagine there could be some damage to the home button. Unfortunately there is no home button fix besides going to Apple and having them replace it. I haven’t heard of it happening with the 8, but the 7 series has had issues with the audio IC which has delayed or prevented startup. It could possibly be something like that as well.

The daughter boards are different throughout each Samsung model. You would need to get a daughterboard specifically made for the s7