Getting in Touch
Videos doesn't have a mailing list, but you can get in touch with the developers either by writing on a more general purpose GNOME mailing list (firstname.lastname@example.org is probably appropriate).
Before you want to file a bug, make sure you are running the latest version. After that, you must find out if the bug is caused by Videos or by its backend (GStreamer). If it is something visual in the video or something in the audio, it probably is a bug in the backend and has to be submitted to the right bug tracker. To see what backend you are using check the "About box" of Videos.
Backend: GStreamer bug
If you are running the GStreamer backend, and you notice a problem with the sound or video (out of sync, garbled output, …), you can file a bug to GStreamer. As with all bugs, first check if it isn't already reported. If your bug isn't in that list, file new bug. You might want to include the output of this command to your bug report:
GST_DEBUG_NO_COLOR=1 GST_DEBUG=*:2 totem 2> log
or if the problem seems to be with the file you're playing, include the output of:
gst-launch-1.0 playbin uri=file://<location of the file> video-sink=cluttersink audio-filter=scaletempo
If possible, please attach a snippet of the file which reproduces the problem.
If you are convinced the problem is with Totem itself, you should first check the bug list if your bug isn't already reported. If you can't find your bug, go ahead and file it on the the Totem bug tracker. To have better debug-output, open a terminal and enter the following command. G_MESSAGES_DEBUG=all totem
Git: git clone https://gitlab.gnome.org/GNOME/totem.git
Videos follows a design-based approach for new features. The current iteration of the designs can be found on this page