I've worked on hours on iphones that have boot up issues and some can be cleared by connecting to iTunes and some can't. Its a software glitch that can't be repaired no mater if the battery is replaced screen or anthing else. At that time its time to replace the phone.
If you tried going into Recovery Mode the last resort is DFU Mode: I have attached an artctle that explains and also helps you to do that just click on the link.
https://www.ikream.com/2018/02/how-to-fix-apple-iphone-6-boot-loop-blod-and-other-power-related-issues-troubleshooting-guide-23912