Sources of FLACs for the best quality for separation process

Introduction

Don’t use YouTube or mp3 as input files for separation. Compression decreases the quality of the output. If you need music video, Tidal has videos up to 320 kbps AAC audio, with bitrates varying by content and quality settings. None of the online rippers download videos from Tidal. You need a valid subscription and local ripper for it (more later below).
If you're forced to use YT, download audio, preferably as Opus if it's available for your video, and it exceeds 16kHz on spectrogram (AAC might be better up to 16kHz).
To enhance results from YT by combining Opus and AAC read
If you want to verify if your input file is really lossless:

https://fakinthefunk.net/en (sometimes streaming services share bad/lossy versions)

For output files, don’t set quality to mp3 in Colabs (export your separations as WAV/FLAC) - YT will recompress it

To upload your lossless output file also losslessly on YT to avoid recompression read
Masters on streaming services can be different from those released on CD - esp. the pre-streaming era albums. Changes might be sometimes slight (minor differences in iLUFS which also mp3 will show) or big in volume (more or less dynamic masters, or sometimes straight up remasters without any notice)
If you have to use YT as input read here

__Sources of FLACs for the best quality for separation process__