What is a simple video editor for Linux and Windows that allows me to cut small parts out of large video files, and make them into something postable on Lemmy?

Ffmpeg?

Ffmpeg -ss (start time) -i (inputFile) -c copy -t (clip length) (outputFile.Name)

Avidemux would work. I often use it because, as long as you cut at keyframes, you don't have to reencode anything (so it's fast).

check out losslesscut

kdenlive is nice but maybe overkill for your use case