We are currently struggeling to get to connect a Linux (Pine) phone to a HDMI monitor. Very bad experience with micro-usb to HDMI adapter. We tested three of those, not all on the PinePhone, none of them worked.
Only one worked a long time ago, and was gone then and not replaceable. We bought a Google Chromecast 3, which worked right away with the Google apps from the Google play store. Unfortunately, big brother Google is always watching what you are watching on the screen.
But what are the alternatives? A Chrome browser installed on a Windows PC could stream to a Google Chromecast 3 without additional plugins. Unfortunately, whe could not find an installation instruction for Chrome for the PinePhone PostmarketOS. (We only go back to Ubuntu Touch for the PinePhone, if LibreOffice works on it.) But a Chrome browser could be an easy way to mirror your phone to a HDMI monitor when connected to a Google Chromecast.