my macbook wont start after I tried to update to big

Since it functions din Safe Mode is an indication of some other possible Issues underlying the problem.

1 - Third Party Software that gets load when loading in Normal Mode. Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation. The Built-in Security of Big Sur is all that is required

1A - Example - Cleaners like CleanMyMac (CMM ) is a prime example of a Cleaner with disasterous results. A fast search of the ASC Support turns up over 600 hits all pointing to Removal ASAP. https://discussions.apple.com/search?q=cleanmymac

2 - Lack of Empty Space - one should always keep at least 15% of the Total Drive Capacity as Empty Space for the OS and computer to function without issues. Example - a 128 GB Internal Drive x 15% = 19.2 GB As Empty Space.

Start up your Mac in safe mode this may or may not allow the update / upgrade process to complete.

Then it seems the update was successful ??

Had the same problem, after updating to Big Sur from Catalina my iMac 2020 got stuck at 90%, I tried so many things but nothing happened.