Big Sur Installer: "An error occurred loading the update."

FYI, there is an ongoing network outage impacting CDN (Content Delivery Networks) Akamai especially as well as Level3 backbones, Cloudflare and other DNS providers that started yesterday and continues this morning. This might be messing with your downloads. Try setting DNS manually on the Mac to 8.8.8.8 and 8.8.4.4 while downloading the installer.

This Mac has a T2 security chip. Make sure you login to the Recovery Console and allow USB Booting with a local admin account.

If Catalina via USB worked then boot into Catallina and re-download Big Sur via Terminal

sudo softwareupdate --fetch-full-installer --full-installer-version 11.5

Then erase the USB flash drive and name it MyVolume and re-burn it via Terminal

sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

Try running the installer via this method first. It will prompt to quit Terminal when it is ready to reboot.

sudo ./Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/startosinstall --agreetolicense --nointeraction

If that fails, try the USB installer again and completely erase everything from your internal disk with Disk Utility and allow the installer to create the volumes.

There will likely be firmware updates being made to the T2 and the Mac may appear completely turned OFF, it is not OFF it's writing to the T2 SoC and upgrading the BridgeOS. Do not interrupt it. It may reboot more than once.

If all else fails you may need a second Mac a Thunderbolt cable and Apple Configurator 2 on the second Mac to revive the T2 chip in the MacBook Pro 2019. Apple's very specific instructions on how to do this:

https://support.apple.com/guide/apple-configurator-2/revive-or-restore-an-intel-based-mac-apdebea5be51/mac

The USB-C charging cable is NOT a Thunderbolt data cable you will need a real Thunderbolt3/4 USB-C cable.

Suggest downloading the Application Etrecheck directly from a Trusted Developer and well Respected ASC Contributor. The application is free or paid from added features. Run the application with Full Disc Access ( Security & Privacy - Full Disc Access ). It will take a Snap Shot -  both the hardware and software. The Report will Not Reveal Any Personal Information. Post back the Full Report - copy and paste - using the Additional Text Icon ( 3rd Icon to last )

We can have a look at the report for possible issues and may have possible suggestions to resolve the issues.

Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.

Similar issue over here >> https://discussions.apple.com/thread/252982698 with similar question. Is the Internal Drive original Apple of Aftermarket ?