I have different video resolutions: 1920 x 1080 and 3840x2160.
I noticed the quality of 1920x1080 is not that good, so if possible I want to maintain better quality.. my question is:
Can I make 1 video with different video resolutions, that converge into one video with good quality (to be seen on YT)? If it is yes, how can I set the video in this "new project" and also in "render as"?
Thank you
You can't make a file look better than it actually is. If some of your source is HD media you can set the project to HD and then render to HD (1080p). The UHD media will look a bit better when downscaled to HD. To compensate for that you could try to use sharpening on the HD footage to fake additional detail.
Alternatively you could try to upscale the HD footage using the Fx "smart upscale" on the media (right click on media, Media Fx). I'd still render to 1080p personally. You could try a UHD render but it might look soft.