Avdija was born on kibbutz Beit Zera, Israel. He played association football until he entered fourth grade, when he began focusing on basketball, the sport his father played professionally. Avdija initially competed at the youth level for Bnei Herzliya, and in 2013, he joined the youth ranks of Maccabi Tel Aviv where he played under the coach Shai Omer. From 2017 to 2019, Avdija led Maccabi Tel Aviv to three consecutive Israeli youth state championships. In August 2018, he participated in Basketball Without Borders Europe in Belgrade, where he was named camp MVP.

On 5 November 2017, Avdija started his professional career with Maccabi Tel Aviv, signing a six-year deal with the club. On 19 November, he made his professional debut in the Israeli Premier League, playing three minutes against Ironi Nes Ziona. At 16 years and 320 days of age, he became the youngest player to ever play for his club’s senior team. Avdija made his EuroLeague debut on 22 November 2018, at the age of 17, in a 74–70 loss to Fenerbahçe. He scored two points in three minutes of playing time.

In April 2018, Avdija averaged over 17 points and nine rebounds for Israel at the Albert Schweitzer Tournament, an under-18 competition in Mannheim. In July 2018, he played at the FIBA U20 European Championship in Chemnitz, leading Israel to a gold medal. Avdija averaged 12.7 points, 6.4 rebounds, 1.1 assists and 1.3 steals per game, while earning a spot on the all-tournament team with teammate Yovel Zoosman. Later that month, Avdija competed at the FIBA U18 European Championship Division B in Skopje. Appearing tired from his previous tournament and lacking in endurance, he averaged 17.3 points, 7.4 rebounds, 3 assists and 1.4 steals per game.

In January 2019, Avdija played for Maccabi Tel Aviv’s U18 team at Adidas Next Generation Tournament (ANGT) in Munich. He was selected to the all-tournament team after leading the event with 24.3 points, 6 assists and 3.8 steals per game and a Performance Index Rating (PIR) of 31.5. Avdija also ranked second among all players with 11 rebounds per game, while helping his team finish in second place. In February, at NBA All-Star Weekend in Charlotte, North Carolina, Avdija was named MVP of the Basketball Without Borders Global Camp. In May, he joined Maccabi Tel Aviv’s U18 team for the ANGT Finals. He collected all-tournament team honours after leading the event with 24.7 points and 12 rebounds per game with a PIR of 29.7, while ranking second with 6.7 assists per game.

On 24 October 2019, at the age of 18, Avdija made his first start in the EuroLeague, recording six points and three rebounds in 16 minutes in a 76–63 victory over Valencia. On 11 January 2020, Avdija recorded a then career-high of 22 points, shooting 9-of-15 from the field, and five assists in a 94–83 win over Hapoel Holon. On 31 January 2020, Avdija was named Israeli Player of the Month in the Israeli League after averaging 14.5 points, 5.3 rebounds and 2.5 assists per game, with an 18.3 PIR, in four games played that month. On 1 February 2020, Avdija established a new career-high of 26 points, shooting 9-of-12 from the field, and grabbed six rebounds in an 86–81 loss to Hapoel Eilat. Six days later, he scored 13 points, including eight in the second quarter, in a 78–77 victory over Fenerbahçe. His dunk over Luigi Datome during the game would be named EuroLeague Magic Moment of the Season in July 2020. On 16 April, Avdija declared for the 2020 NBA draft.

In July 2019, Avdija led Israel to a second straight gold medal at the FIBA U20 European Championship in Tel Aviv. He averaged 18.4 points, 8.3 rebounds, 5.3 assists, 2.4 blocks and 2.1 steals per game, earning MVP and all-tournament team accolades, as the second-youngest player at the event. Avdija tallied 26 points, 11 rebounds and 5 steals against France in the semifinal, before recording 23 points, 7 assists, 5 rebounds and 3 blocks against Spain in the final.

On 21 February 2019, Avdija made his debut for the Israeli senior national team, in an 81–77 win over Germany, during the 2019 FIBA World Cup qualification stage. On 24 February 2020, he recorded 21 points and eight rebounds in 24 minutes in an 87–63 victory over Romania during the EuroBasket 2021 qualifiers.

Avdija holds dual citizenship of Israel and Serbia, the latter because his father is a Serbian citizen. When he turned 18 years old, he received a deferment from mandatory service in the Israel Defense Forces (IDF) due to his basketball career. On 1 April 2020, while the basketball season was suspended as a result of the COVID-19 pandemic, Avdija was drafted into the IDF for a short service.