If I have understood correctly, then you could use the following formula, assumptions per tags there is no Excel Constraints therefore the given formulas should work :

=UNIQUE(VLOOKUP(TOCOL(D3:E3,1),O$3:P$15,2,0))

Or, If not using Structured References aka Tables then using BYROW() spill it for the whole data :

=BYROW(D3:E15,LAMBDA(α, UNIQUE(VLOOKUP(TOCOL(α,1),O3:P15,2,0))))

NOTE: The above formulas will return Unique Values, since some of the rows have double INPUT data, so if you want them to concatenate then in place of UNIQUE( replace it with TEXTJOIN(", ",1, however, which i assume is not needed. MS365 Exclusive!!

Explanations:

• Firstly, using TOCOL() function, we could have also used TOROW() but completely missed it, anyways. Using TOCOL() it converts the 1x1 data into a single column array as well as excludes if any empty cells.
• Secondly, using VLOOKUP() function to grab the Nation column from the other range based on the Lookup_Value we have to get an Exact Match.
• Finally, using UNIQUE() function to exclude the duplicates as both Accounts has same Nation. Here I have assumed one would need the duplicates as well as one is not asking to join the return outputs. Also, if you intent to use TOROW() then ensure to select the second parameter of UNIQUE() function which is [by_col] thus the formula will be:

=UNIQUE(VLOOKUP(TOROW(D3:E3,1),O$3:P$15,2,0),1)

• Last but not least, if you want to spill instead of drag down for the rest of the cells, then could use LAMBDA() helper function BYROW() which does the same operation as above but ensure to do it for each row to return one single output as an array, however from the screenshot in your OP, it won't work as it seems you are using Structured References aka Tables as for now it doesn't support #SPILL features. Hope it explains well.