A multi-band compressor is useful for mixing & mastering.
Audacity is shipped with a single-band compressor.
Here are 7 free multi-band compressor plugins which work in Audcaity on Windows …
G-Multi … GVST - GMulti
KSHMR Essentials Kick … KSHMR Essentials Kick | W. A. Production
Puncher2 lite … W.A. Production Releases FREE Puncher 2 Lite Transient Shaper - Bedroom Producers Blog
OTT (which is OTT, i.e. extreme) … https://xferrecords.com/freeware
Rough Rider Pro (which is like OTT, but more controls) … https://youtu.be/eKu7fJdQZlU?t=519
ToneBoosters FIX … https://youtu.be/5fffEP5-N5o?t=133 (unusually can expand as well as compress)
ToneBoosters Broadcast … Free (v3) ToneBooster's plugins - #3 by Trebor