Also, its not possible to write your own DSI/MIPI drivers for a phone LCD that you can buy on eBay, partly for not having the DSI/MIPI specification unless you pay upwards of thousands of dollars a year and Raspberry Pi doesn't allow access to the DSI/MIPI HAL layer - its part of the SOC which Broadcom controls.
About the only way to make a device with an LCD is to use the built-in HDMI.
This is a theme with the RPi. They generally implement new hardware in their proprietary firmware first, for lack of skills/time to do a proper upstream job of it (or the convience of copy-pasting some vendor code), then pay someone to develop an open-source driver. For example, here is a massive 80 patch series nearing merge for the new display pipeline in the RPi 4:
About the only way to make a device with an LCD is to use the built-in HDMI.