I'd like to have a bigquery query code to join and calculate 2 tables

I have 2 tables in Bigquery and I'd like to merge/join them and doing some calculation.
Here are the tables :

Table A

Table B

The score on Table A is the total score of games of Table B. However, not all games on Table B is identify.
So, the final table should look like :

Combined

I've tried all the joining statements possibles (I know) but the output is not as desired.

The best I did found was a union all (or distinct) with this query :

select
ID,
Name,
null as Games,
Score
from Table A
Union ALL
Select
ID,
Name,
Games,
Score
from Table A

and here is the output :

The score is not the total of table A - Sum (of Games) score of Table B as expected.

Could you please help me out ?

Thanks

I'm trying to joing query statement between table A and table B and have the score of table A to be the difference from the sum (table A) - sum (table B) for a null "games"

I've tried the following query :

select
ID,
Name,
null as Games,
Score
from Table A
Union ALL
Select
ID,
Name,
Games,
Score
from Table A

Consider below simple approach

select id, name, games, 
  if(not games is null, score, 2 * score - sum(score) over(partition by id, name)) as score
from (
  select * from tableB union all
  select id, name, null, score from tableA
)

if applied to sample data in your question - output is