There is an App to Web solution, that allows you to pass the ECID from your mobile app to your website (when people click on links inside your app to your website).
Unfortunately, this does not work in reverse (web to app), and since I don't know if the user had a previous ECID on web, if that would get stitched to being the same user, or basically you overwrite the user identification for web. It's also possible that if the website opens in a webview that the cookies are kept unique and separate from using the website normally in your browser.
Because of the limitations of this solution, and the fact that we have minimal webviews in our apps, it's not a solution I have investigated much.
Here is the documentation if you choose to pursue this investigation:
https://experienceleague.adobe.com/en/docs/experience-platform/web-sdk/identity/id-sharing
As @bjoern__koth mentioned, there is also the Cross-Device-Analytics, however, this is no longer sold to Adobe Analytics clients... if you already have it, great... if not, the only way to not get access to it is by getting CJA....