if it is just encoding/decoding ffmpeg can go entirely from stdin to stdout IIRC, provided formats support it. So for example it won't work for mp4 output as it needs to seek to write some headers (again, IIRC) when saving it.
I was hoping to combine lengthy h264 .mts video files chopped at 4GB by the video camera into one YouTube upload without duplicating on disk first.
Edit: also maybe some simple filters/ML to detect anything interesting to clip and merge into a time lapse.
I hit that behaviour when I tried to upload encode directly to S3 bucket, turned out only some formats worked
You would have to set a stream up in the Youtube creator web page, in order to get a streaming key and with it, a full RTMP URL.
Then in FFmpeg, use "-f rtmp://...your target url" as output. Youtube will stream a live video (which you can hide as "unlisted") and afterwards, the video will remain in the channel as a VOD.