It supports FLAC obviously but also Opus. For my phone, I've ripped my FLAC files to Opus and can carry my entire music collection wherever I go. I used their import tool to build the same folder structure that I have on my NAS. I have iTunes installed on my Windows machine (no Macs at home) but I try to avoid it if at all possible.
There's a growing trend these days of making every single player app into spyware, and it's sad. I won't use reader apps or player apps that read or play local files that are going to transmit my activity off device for no reason that benefits me.