Relocated items after macOS update to 11.3.1
To your question, which file is the modified one, the document you've added already explains that:

Configuration files with the suffix &quot;system_default&quot; were edited or
customised but the changes were allowed to remain installed

...which means your current file &quot;group&quot; is the modified one and &quot;group.system_default&quot; is the default one, but wasn't installed, because the changes to &quot;group&quot; were allowed.

_trustd:*:282:_trustd

...is a user of the same named group on your computer, as also found in &quot;Directory Utility.app&quot; (for which you can search in Spotlight).
&quot;282&quot; is the &quot;PrimaryGroupID&quot;.
The man pages for trustd mention:
trustd(8)                 BSD System Manager's Manual                trustd(8)

NAME
     trustd -- Daemon and LaunchAgent that performs trust evaluations

SYNOPSIS
     trustd

DESCRIPTION
     trustd provides services for evaluating trust in certificates for all processes on the system.

     This command is not intended to be invoked directly.

BSD                               May 7, 2021                              BSD

Who initiated the changes, or why, I cannot say - I would think you yourself should have a better idea, if you've done any modifications, or which application you've installed, that could have changed the &quot;group&quot; file.
Or maybe somebody else here knows.
About the date: if you look through the system volume, you may find that pretty much all system files have the same modification date (which is new in Big Sur with its boot snapshot).