These are covers of mostly VOCALOID songs, made with the vocal synthesis platform UTAU. (What is UTAU?)
The off-vocal version of these songs are published by their artists. I then use UTAU, plus the voicebanks, to create the vocals; this is called tuning. I then mix the tuned vocal-only audio into the off-vocal, hopefully not screwing up the mix in the process. Then it's combined with the original video (or with videos that I made), then published to YouTube and niconico.