SQLite 3.27

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
CREATE TABLE leagues_table (
  `League` VARCHAR(16),
  `Team` VARCHAR(15),
  `GF` INTEGER,
  `GA` INTEGER,
  `Points` INTEGER
);
INSERT INTO leagues_table
  (`League`, `Team`, `GF`, `GA`, `Points`)
VALUES
  ('Champion Group A', 'Bayren', '18', '5', '16'),
  ('Champion Group A', 'Atlteico Madrid', '7', '8', '9'),
  ('Champion Group H', 'PSG', '13', '6', '12'),
  ('Champion Group H', 'RB Leipzig', '11', '12', '12');
SELECT league, team, gf, ga, points
FROM (
  SELECT *, ROW_NUMBER() OVER (PARTITION BY league ORDER BY points DESC, gf - ga DESC) rn
  FROM leagues_table
)
WHERE rn = 1


League
Team
GF
GA
Points


Champion Group A
Bayren
18
5
16

Champion Group H
PSG
13
6
12


SELECT lt1.* FROM leagues_table lt1
WHERE NOT EXISTS (
  SELECT 1 FROM leagues_table lt2
  WHERE lt2.league = lt1.league 
    AND (lt2.points > lt1.points 
         OR (lt2.points = lt1.points AND lt2.gf - lt2.ga > lt1.gf - lt1.ga)
        ) 
)


League
Team
GF
GA
Points


Champion Group A
Bayren
18
5
16

Champion Group H
PSG
13
6
12


SELECT lt1.* FROM leagues_table lt1
WHERE lt1.rowid = (
  SELECT lt2.rowid
  FROM leagues_table lt2
  WHERE lt2.league = lt1.league
  ORDER BY lt2.points DESC, lt2.gf - lt2.ga DESC
)


League
Team
GF
GA
Points


Champion Group A
Bayren
18
5
16

Champion Group H
PSG
13
6
12