Usually, it’s some highlight and shadow control, plus tweaks in contrast and clarity.
There are packages from the likes of Adobe, Affinity, Capture One, DxO, Luminar and Topaz. Some are available on subscription, others are outright purchases and there are free/shareware options too. Most of my mono conversions are done through Silver Efex from DxO’s Nik Collection which I use as a plug-in in Adobe Lightroom. I will apply some Lightroom adjustments before taking the file into the plug-in. Usually, it’s some highlight and shadow control, plus tweaks in contrast and clarity.