Apple has stopped signing iOS 13, so it might be harder to install iOS 13, the iPhone 6 Plus will recommend iOS 15.5 currently. Custom iPSW from Apple can be install on a Mac, or Windows computer through iTunes by shift, right-clicking the Restore-iPhone button. These iPSW must be signed by Apple to restore to custom iPSW or else a error will return. Currently the only signed software for iPhone8,2 (iPhone 6+) is iOS 15.5 (19F77). The only current way to do it is via 3rd-party software, which can damage the iPhone or having unauthorized version of iOS. This is not recommended. SHSH2 blobs are required which are unique to the iPhone.

On iOS 15, try going to Settings, Safari, Advanced, Experimental WebKit Features, then Turn "HDR Media Capabilities, Media Capabilities Extensions, Media Session API" off, then re-enable it, Then restart Safari. If not you can go under "Advanced" tab in Safari settings, and click "Remove All Website Data". NOTE, doing this will clear data, that could be used for tracking, but can be used by websites to save login-information, and to speed the websites connection speeds. Also Clearing Ram on iPhone may help by entering the shutdown menu, then Tap, and Hold the Home Button for about 5 seconds until you return to home-screen.