Here's a load of techniques that I started with as the "classic" ways to modify STLs, particularly handy for scenery.
War Scenery Kickstarter: https://www.kickstarter.com/projects/warscenery/fall-of-castograd-3d-printable-grimdark-terrain-and-minis?ref=7tlluv
War Scenery Website: https://www.warscenery.com/
Support the channel on Patreon and get videos a week early: https://www.patreon.com/ArtisansofVaul
Free Add Ons
Bagapie:
RePrimitive: https://github.com/eXzacT/RePrimitive
Modifier List: https://extensions.blender.org/add-ons/modifier-list-fork/
Quick Snap: https://github.com/JulienHeijmans/quicksnap
CharMorph: https://github.com/Upliner/CharMorph
Simple Tabs: https://chippwalters.gumroad.com/l/simpletabs
Edge Flow: https://github.com/BenjaminSauder/EdgeFlow
Sculpt Bridge: https://blendermarket.com/products/sculpt-bridge-tool
(Affiliate links) SUPPORT THE CHANNEL BY BUYING SOME STUFF YOU WERE GOING TO GET ANYWAY ;p
Machin3 Tools: https://superhivemarket.com/products/machin3tools/?ref=834
nSolve: https://superhivemarket.com/products/nsolve/?ref=834
Hard Ops and Boxcutter discount bundle: https://superhivemarket.com/products/hard-ops--boxcutter-ultimate-bundle/?ref=834
Hard Ops: https://superhivemarket.com/products/hardopsofficial/?ref=834
Boxcutter: https://superhivemarket.com/products/boxcutter/?ref=834
Resample Mesh: https://superhivemarket.com/products/resample-mesh/?ref=834
Mesh Machine: https://superhivemarket.com/products/meshmachine/?ref=834
Solidify Plus: https://superhivemarket.com/products/solidify-plus/?ref=834
Mass STL Exporter: https://sonesson.gumroad.com/l/mass-stl-exporter
Just Panels: https://superhivemarket.com/products/just-panels/?ref=834
Construction Lines: https://superhivemarket.com/products/construction-lines/?ref=834
Cablerator: https://superhivemarket.com/products/cbl/?ref=834
Mesh Copier: https://superhivemarket.com/products/mesh-copier/?ref=834
Grid Modeler: https://superhivemarket.com/products/grid-modeler/?ref=834
Curve Machine: https://superhivemarket.com/products/curvemachine/?ref=834
Decal Machine: https://superhivemarket.com/products/decalmachine/?ref=834
Punch It: https://superhivemarket.com/products/punchit/?ref=834
One Click Damage (OCD)/Cracker discount bundle: https://superhivemarket.com/products/ocd--cracker-damage-bundle/?ref=834
One Click Damage (OCD): https://superhivemarket.com/products/ocd/?ref=834
Cracker: https://superhivemarket.com/products/cracker/?ref=834
Flowify: https://superhivemarket.com/products/flowify/?ref=834
Simple Bend: https://superhivemarket.com/products/simple-bend/?ref=834
Conform Object: https://superhivemarket.com/products/conform-object/?ref=834
Curves to mesh: https://superhivemarket.com/products/curves-to-mesh/?ref=834
Mesh Materializer: https://superhivemarket.com/products/mesh-materializer/?ref=834
Favourite modifiers: https://superhivemarket.com/products/favourite-modifiers/?ref=834
Wrap Master: https://superhivemarket.com/products/wrap-master/?ref=834