His father, Magnus Holtz (born Andersson), is a former ice hockey player who played two seasons for Västerviks IK.

[4] He was named for the Stockholm South team in the national junior hockey tournament TV-pucken in 2017 and scored 6 goals and 10 points in 6 games.

[8][9] He recorded his first SHL point in the league premiere on 14 September when assisting Michael Haga to the 1–1 equalizer in the 4–2 win against Linköping HC.

Following the conclusion of the NHL pre-season, Holtz was assigned to the AHL's Utica Comets, where he finished among the top rookie scorers, recording 26 goals and 51 points in 52 games.

[14] In one of a few brief stints with the Devils, Holtz scored his first NHL point, an assist on a goal by Pavel Zacha against the Florida Panthers on 9 November 2021.

[18] On 29 June 2024, during the 2024 NHL entry draft, Holtz and Akira Schmid were traded to the Vegas Golden Knights in exchange for Paul Cotter and a third-round pick in 2025.