My phone specs say front camera does 720p max. Open Camera is able to record 1080p. For real?

Open Camera has a range of resolutions available for both cameras and for stills and video. I don't know what settings your phone's stock app has, but have you looked to see whether it's got other options? The default video option may not be the highest resolution it's capable of (especially if it has to reduce the frame rate in order to increase the resolution).

Any 8Mp camera has enough pixels to record 1080p. Whether it has the bandwidth to do so is a different question. I'd not myself make any assumption that all such cameras can do it, since 8Mp is only one part of the hardware spec.

 

Hmmm, maybe check your sources, according to Blu's site your X5 should be able to provide 1080p@30fps for both the main back and selfie front cameras:
https://bluproducts.com/devices/vivo-xl5-x5/
https://www.gsmarena.com/blu_vivo_x5-9853.php

Regarding that limitation in the default camera app, obviously there's a discrepancy between the actual hardware and the app Blu has provided. But outside the default camera apps in flagship models from Google or Samsung (that might involve proprietary tie-ins), just personally I've found Open Camera to be a better option most of time anyway. It has a really expansive feature set so you have a lot more customization and optimization options.

 

No, it varies by maker and version. It will come down to trading one or the other for added performance or features. Checkout github for your device and find the packages apps and settings platform and click on commits, as you scroll you will see where these things are switched back and forth or changed becuase it did not work the way the DEV intended. Hope that helps

 

https://github.com/androidtrackers/certified-android-devices.git 530 something commits in there, you should be able to find commits and changelogs for your phone