Can I use whatever music I want in YouTube shorts under 60 seconds?

The plain truth about it: The absolute truth: You will be able to use the song without any problems only when adding it from the Add Soun library of YouTube. If you edit the song within your video editor software (like CapCut or Premiere), then the copyright detection syst will immediately blacklist your channel or steal money from you for that little 5-second-long song=

I also tried it for a while and found no problems.

using some one elses muic without permission is a nono

stick with licensed music on your Shorts

Use licensed music or YouTube's music; don't go for TikTok.