In SQL Server, what's the best way to split multiple delimited strings into rows without a function?
Transform the delimited string to a JSON array, and use OPENJSON, which exposes the lexical ordering for arrays.  eg
This
select *
from openjson('[&quot;w&quot;,&quot;x&quot;,&quot;y&quot;,&quot;z&quot;]')

outputs
key       value  
--------- -------
0         w      
1         x      
2         y      
3         z      

So
DROP TABLE IF EXISTS #TestData;

CREATE TABLE #TestData(GroupId VARCHAR(10),
                       Val1 VARCHAR(100),
                       Val2 VARCHAR(100)
                       );
INSERT INTO #TestData
VALUES ('G1', '1,2,3', 'a,b,c'),
       ('G2', '4,5,6', 'd,e,f'),
       ('G3', '1,2,3,4', 'w,x,y,z');

with q as
(
  SELECT GroupId, concat('[&quot;',replace(Val1,',','&quot;,&quot;'),'&quot;]') Val1,concat('[&quot;',replace(Val2,',','&quot;,&quot;'),'&quot;]') Val2
  FROM #TestData
)
select GroupId, v1.value Val1, v2.value Val2
from q
cross apply openjson(Val1) v1
cross apply openjson(Val2) v2
where v1.[key] = v2.[key]

outputs
GroupId    Val1     Val2
---------- -------- ---------------
G1         1        a
G1         2        b
G1         3        c
G2         4        d
G2         5        e
G2         6        f
G3         1        w
G3         2        x
G3         3        y
G3         4        z