[2] NFL.com described Rapp as a player who has "beaten long odds, undeterred by a city that failed to support prep football, a high school community that openly mocked him, coaches who were unprepared to develop his talents, college recruiters who blatantly overlooked him and kids who racially taunted him for his Chinese ethnicity.
[5] He committed to Washington in the spring of his junior year, choosing the Huskies over several Football Bowl Subdivision programs, including Oregon, Stanford, Nebraska, and Notre Dame.
[7] He received an invitation to play in the prestigious U.S. Army All-American Bowl game, but elected not to attend as it interfered with his schedule following his early enrollment at the University of Washington.
[25] On September 8, 2019, Rapp made his professional regular season debut in the Los Angeles Rams' season-opener at the Carolina Panthers and recorded eight combined tackles (four solo) during a 30–27 victory.
On October 16, 2019, starting strong safety John Johnson was placed on injured reserve for the rest of the season with Rapp subsequently being promoted to his position in his absence.
With 58 seconds left in the game on third and 16, Rapp and Jalen Ramsey made an error in coverage resulting in a 46-yard completion to wide receiver Emmanuel Sanders, leading to a game-winning field goal that knocked the Rams out of playoff contention.
[27] The following week, Rapp made six combined tackles (five solo), an interception, and had his first career fumble recovery on a mishandled snap by Kyler Murray during a 31–24 win against the Arizona Cardinals.
[28] He finished his rookie season with 100 combined tackles (62 solo), eight pass deflections, three interceptions, one fumble recovery, and a touchdown in 15 games and ten starts.
[29] Rapp entered training camp as a candidate to become the starting free safety after Eric Weddle opted out of the 2020 NFL season due to the COVID-19 Pandemic.
On October 26, 2020, Rapp recorded seven combined tackles (three solo) and had his lone interception of the season on a pass thrown by Nick Foles to wide receiver Darnell Mooney as the Rams defeated the Chicago Bears 24–10 on Monday Night Football.
On November 17, 2020, the Los Angeles Rams placed Rapp on injured reserve due to his knee injury and missed the last seven games of the season (Weeks 11–17).
Morris held an open competition between Rapp, John Johnson III, Nick Scott, Terrell Burgess, Jovan Grant, Troy Warner, and Paris Ford.
[35] On September 12, 2021, Rapp started in the Los Angeles Rams' season-opener for the first time in his career and made ten combined tackles (six solo) during a 34–14 win against the Chicago Bears.
On December 19, 2022, Rapp made three combined tackles (one solo), one pass deflection, and sealed the Rams' 17–16 victory over the Las Vegas Raiders after intercepting quarterback Derek Carr in final seconds of the game.
[41] The following week, he recorded seven combined tackles (six solo), one pass deflections, and had his second interception of the season against, Aaron Rodgers during a 12–24 loss at the Green Bay Packers.
Rapp entered training camp as the clear cut third option at safety with Jordan Poyer and Micah Hyde cemented as the starters.
[44] In the second quarter of the Week 11 game against the New York Jets, he suffered a neck injury after colliding with running back Breece Hall with Rapp ultimately being carted off the field in an ambulance.
On December 23, 2023, Rapp started in place of Micah Hyde and collected a season-high nine combined tackles (seven solo) during a 24–22 victory at the Los Angeles Chargers.
Entering training camp, Rapp was projected to earn one of the starting safety positions, but received competition from Damar Hamlin, Mike Edwards, Cole Bishop, and Kareem Jackson.
[51] During the Bills' week 4 matchup against the Baltimore Ravens on NBC Sunday Night Football, Rapp suffered a concussion while tacking running back Derrick Henry,[52] causing him to miss the next game.