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=LAMBDA(range1,range2,default,
LET(
rows1,ROWS(range1),
rows2,ROWS(range2),
cols1,COLUMNS(range1),
cols2,COLUMNS(range2),
rowindex,SEQUENCE(rows1+rows2),
colindex,SEQUENCE(1,MAX(cols1,cols2)),
result,
IF(
rowindex<=rows1,
INDEX(range1,rowindex,colindex),
INDEX(range2,rowindex-rows1,colindex)
),
IFERROR(result,default)
)
)