Excel: VLOOKUP/TEXTJOIN to get comma-separated result in cell for multiple matches in comma-separated cells in a column

Here is one way you could try using TEXTJOIN()+FILTER()+REGEXTEST()+MAP()

=MAP(A8:A11, LAMBDA(x, TEXTJOIN(", ", 1, FILTER(A2:A5, REGEXTEST(", "&B2:B5&", ", ", "&x&", ")))))

Or,

=TEXTJOIN(", ", 1, FILTER(A$2:A$5, 1-ISERR(SEARCH(", "&A8&", ", ", "&B$2:B$5&", ")), ""))

With Office 365 functions the below formula

• a contains the unique values of Test ID
• b execute a LAMBDA function on the a elements, which collects the Function ID of the corresponding row where the Test ID is found
• Last row generate the result table display the two columns side-by-side.

=LET(a,UNIQUE(TEXTSPLIT(TEXTJOIN(",",TRUE,B2:B5),,",")),
b,MAP(a,LAMBDA(x,TEXTJOIN(",",TRUE,IF(IFERROR(SEARCH(","&x&",",","&B2:B5&","),0),A2:A5,"")))),
HSTACK(a,b))

Here is a different approach using custom LIKE function using LAMBDA().

=TEXTJOIN(",",1,FILTER($A$2:$A$5,LIKE($B$2:$B$5,"*"&E2&"*")))

LIKE function signature is as following-

LIKE =LAMBDA(range,criteria,[and_or],LET(fnλ,IF(TYPE(criteria)=64,LET(and_or,IF(ISOMITTED(and_or),OR,and_or),LAMBDA(rng,and_or(COUNTIF(rng,criteria)))),LAMBDA(rng,COUNTIF(rng,criteria))),BYROW(range,fnλ)))