>>1002818>-crf, b:v, and threads do. Or how the scale works.-crf is a "constant rate factor". What it does exactly depends on the encoder, but think of it as another though less important quality control. Lower means better quality. Personally I set it to 10 and never touch it.
-threads says how many CPU cores the encoder is allowed to use. More threads will make encoding quicker but put more strain on your system.
The scale video filter does what it says on the tin. It resizes the video to the resolution x:y where -1 is a wildcard. So -1:360 resizes to a height of 360 pixels aka 360p. The width will be whatever it takes to preserve the aspect ratio.
Just have a template and then deviate as necessary. I find it easier to delete stuff, so I keep a bunch of filters in my template which I sometimes use
ffmpeg -i "input.mp4" -ss 00:00:00.000 -to 00:00:00.000 -c:v libvpx -threads 4 -crf 10 -qmax 55 -b:v 800K -filter_complex "crop=in_w:in_h:0:0,scale=-1:480:flags=lanczos;afade=t=in:st=0:d=0.3" -c:a libvorbis -ac 2 -qscale:a 1 -metadata title="" -y "output.webm"
Apart from edge cases you should use 2pass for your final encode. That is a mode where the whole file is analyzed on pass 1 so the encoding on pass 2 can perfectly allocate bitrate to where it is needed.
For it to work you remove all the quality controls (-crf, -qmax, etc) except -b:v and add -pass 1, then run it again with -pass 2.
In the early days of webm on 4chan someone wrote an extensive thread on /wsg/ which made ffmpeg accessible to a lot of people. The basics should hold up even 6 years later if you want a simple intro. There are no archives from the time and only a screencap survives (picture related), but the author later published a slightly updated text-only backup so you can copy stuff: justpaste [dot goes here] it/6nvv0
