This thread started as a docker thread. If you've installed Jellyfin directly on Debian, the directory that trickplays are stored in is /var/lib/jellyfin/metadata/library.
Jellyfin 10.10.7 (Docker)
Ubuntu 24.04.2 LTS w/HWE
Intel i3 12100
Intel Arc A380
OS drive - SK Hynix P41 1TB
Storage
4x WD Red Pro 6TB CMR in RAIDZ1