I'm having this problem where when I try rendering my animation I keep having background color and I tried googling and youtube tutorials on how to make it transparent but whenever I render my composition (which ofcourse doesn't have solid background and when transparent grid is active I see transparency) I still get background color. I set my render options to QuickTime and color to RGB+Alpha and still get background everytime. Help me please.
I think you might be getting confused. Have you tried re-importing the video back into AE and checking the alpha channel there? Otherwise use your player app's information function to check the pixel format, it should tell you if there's an alpha channel present. MPV (my fave) has this function, as does VLC (tried and tested fave).
Also Quicktime is not a codec, it's a container. Some quicktime-compatible codecs support alpha, some do not. Codecs you could use which do support alpha include prores 4444, DNxHR 444, Cineform, and Apple Animation. Generally these codecs are designed for intermediate use — sending a file to another part of the post production chain — because they're not really optimised for delivery (huge files).