What is it, Help Me out regarding stretching 4:3 to 16:9

I’m New here and i wanted to know something, i recorded my gameplay in 800x600, then edited it using shotcut and made this video https://www.youtube.com/watch?v=L66PYsYk2pY
i dont know how it is even possible to make it.
in 1080p??
Can i know how to do it if its normal