Right, tidal offline files can only be accessed through the tidal app. The best plan is to use android only for doing internet streaming and mostly for fun over SQ or for demoing and finding new albums; although tidal masters is very good SQ and can compete with Mango since it’s higher resolution than normal CD quality. And it’s not like Android sounds bad or something, DX220 flattens many home systems. It’s just that Mango was surprisingly good to my ears, an extra helping of detail and clarity.
Sites like HDtracks, Acousticsounds, various others are good for building a library legitimately. Also buying and ripping CDs with EAC or similar high quality software. It took us years to do this and a lot of money, to do it legitimately at least.
The cheapest way for good sounding music is to use only Android and tidal, favorite as many Masters albums as you can as they unfold to 352khz in many cases. This way you have only CDQ music and beyond, and the vast majority of well known albums are on tidal. Buy a few DSD recordings like Thriller and play back in Mango to show off what the 220 can do and for demoing headphones and other gear.