Raw ALSA can only have one program playing sound unless your hardware has support for hardware mixing (this would generally mean it's very old and also very expensive). More likely you are/were using ALSA's dmix plugin which is a small barebones version of the core functionality of pulseaudio or pipewire. They replace dmix but also give you things like per-program volume control, better latency, better power usage, better mixing, bluetooth support, the ability to do fancy things with audio routing without restarting every program playing audio, etc.