This command worked to enable video thumbnails in Dolphin file manager on Linuch Arch / Manjaro:

sudo pacman -S ffmpegthumbs qt5-imageformats kimageformats taglib raw-thumbnailer;cd;sed -i "s|Plugins=|Plugins=ffmpegthumbs,|g" .config/dolphinrc

i assume that on different Linux, one use same or similar packages and different manager from pacman