Id like to know, should I use defaul or rec709 colour space if I will make videos to youtube. Or is it even better choice? I use canon 90d, gopro 5, 11, nikon p950 and dji drones.
What difference are between default and rec709 colour space?
"Color Space" is actually two different things -- Input color space and Output color space.
1. Input color space is the color space you shot.
2. Output color space is the color space you produce, which always remains within the confines (gamut) of the color space you shot, as well as within the color space of your monitor. You cannot make HDR10 (defined in ACES) from REC709, but the opposite conversion is common.
If the color space in step 1 is REC 709 in 8 bit 4:2:0 conventional format, which constitutes 90% of everything that is shot, seen, and produced, you have nothing to worry about. Everything is taken care of for you automatically, IOW don't touch it. In 8 bit Vegas projects, Default and REC 709 are equivalent.
If you have shot something else, post the complete properties using the link above, and someone will guide you as to the possibilities and many pitfalls awaiting you.