Looking at the source, it's simply strcoll().
Jan
Index: bin/expr/expr.1
===================================================================
RCS file: /cvs/src/bin/expr/expr.1,v
retrieving revision 1.24
diff -u -p -r1.24 expr.1
--- bin/expr/expr.1 16 Aug 2017 20:10:58 -0000 1.24
+++ bin/expr/expr.1 1 Feb 2022 22:53:12 -0000
@@ -39,9 +39,10 @@ Returns the evaluation of
if neither expression evaluates to an empty string or zero;
otherwise, returns zero.
.It Ar expr1 Li "{=, >, >=, <, <=, !=}" Ar expr2
-Returns the results of integer comparison if both arguments are
-decimal integers; otherwise, returns the results of string comparison
-using the locale-specific collation sequence.
+Returns the results of integer comparison
+if both arguments are decimal integers;
+otherwise, returns the results of string comparison using
+.Xr strcoll 3 .
The result of each comparison is 1 if the specified relation is true,
or 0 if the relation is false.
.It Ar expr1 Li "{+, -}" Ar expr2
