Where has my iTunes library gone?

Hi all. Some advice would be much appreciated (in simple terms) before I get anymore frustrated. I recently purchased a new 2020 MacBook Air 13". I haven't been onto to iTunes for a while but have also recently started at the gym and wanted to change the music on my phone from all the Christmas that seem to be on there. However, I seem to have no idea how to access my iTunes library. I have thousands of songs that I have bought over the years - lots of money, history and memories - but do not know how to get them. I do not subscribe to Apple Music - and would assume paying £10 a month to access my own music that has already been paid would not be correct - and cannot download any previous versions of iTunes - 2019/2017 - as it says they are not compatible with my Mac. How do I get my music? Please help. Thanks.