I'm using yt-dlp to download youtube video's. These video's are saved as .webm files. Strawberry does not show these files in the collection.

I suspect the backend can play this file, as ffprobe reports the matroska container to contain an opus audio stream:

ffprobe Riton\ x\ Nightcrawlers\ -\ Friday\ \(Dopamine\ Re-Edit\)\ \[feat.\ Mufasa\ \&\ Hypeman\]\ \(It\'s\ Friday\ Then\ Song\)\ \[IHC1Ma6EBbE\].webm 
ffprobe version n6.0 Copyright (c) 2007-2023 the FFmpeg developers
  built with gcc 13.1.1 (GCC) 20230429
  configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-amf --enable-avisynth --enable-cuda-llvm --enable-lto --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libdav1d --enable-libdrm --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libjack --enable-libjxl --enable-libmfx --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librav1e --enable-librsvg --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libsvtav1 --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-libzimg --enable-nvdec --enable-nvenc --enable-opencl --enable-opengl --enable-shared --enable-version3 --enable-vulkan
  libavutil      58.  2.100 / 58.  2.100
  libavcodec     60.  3.100 / 60.  3.100
  libavformat    60.  3.100 / 60.  3.100
  libavdevice    60.  1.100 / 60.  1.100
  libavfilter     9.  3.100 /  9.  3.100
  libswscale      7.  1.100 /  7.  1.100
  libswresample   4. 10.100 /  4. 10.100
  libpostproc    57.  1.100 / 57.  1.100
Input #0, matroska,webm, from 'Riton x Nightcrawlers - Friday (Dopamine Re-Edit) [feat. Mufasa & Hypeman] (It's Friday Then Song) [IHC1Ma6EBbE].webm':
  Metadata:
    ENCODER         : Lavf60.3.100
  Duration: 00:02:49.19, start: 0.000000, bitrate: 4080 kb/s
  Stream #0:0(eng): Video: vp9 (Profile 0), yuv420p(tv, bt709), 1920x1080, SAR 1:1 DAR 16:9, 60 fps, 60 tbr, 1k tbn (default)
    Metadata:
      DURATION        : 00:02:49.149000000
  Stream #0:1(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
    Metadata:
      DURATION        : 00:02:49.188000000

My versions:
$ strawberry --version
Strawberry 1.0.18
$ uname -a
Linux cedric 6.4.3-arch1-1 #1 SMP PREEMPT_DYNAMIC Tue, 11 Jul 2023 05:13:39 +0000 x86_64 GNU/Linux