Youtube may in fact do a few things. It may stream FLV in many cases, but some platforms may not take that, iPhone for example, so they prolly decide on the fly what to send you.

I did notice that Quicktime players can play .f4v files (Flash mpeg4 style). It's possible that youtube sends you .f4v now.

flv, f4v, mov (which means nothing unless you also state the compressor...), mp4, avi,... if you need to experiment, consider the free ffmpeg. All parameters (aurio rate, audio bitrate, frame rate, video bit rate, dimensions,,.....) everything at your fingertips - literlly. If you use it as commend line tool that is. I love using Norepad to edit my conversion scripts and try a few options quickly. Such as:

for %%q in ( 1 2 3 4 5 6 ) do (
  
    ffmpeg -i myrender.mov -s 320x240 -r 24 -b 500k -qscale %%q -f f4v myrender_%%q_toF4V

)

Put this in a file named convert.bat (not .txt) and double-click it to run this batch file.

And it's a very fast converter at that.