Convert mesh to parametric:
https://www.honeypoint3d.com/2016/04/01/converting-mesh-to-parametric-cad/

Blender
https://www.blender.org/
Blender 3d printing toolbox for beginners:
https://www.youtube.com/watch?v=yAQWRsT3QAY

PARAMETRIC MODELING:

Freecad
https://www.freecad.org/

Fusion
https://www.autodesk.com/products/fusion-360/overview?term=1-YEAR&tab=subscription&plc=F360

Onshape
https://www.onshape.com/en/education/
https://www.onshape.com/en/products/free
Teaching Tech playlist about onshape:
https://www.youtube.com/playlist?list=PLpOS1DA53WRE9vTb13N85R6Zr7I-guYBs
3dprint tutorial with inshape:
https://i.materialise.com/blog/en/onshape-3d-printing-tutorial/
Onshape mobile tutorials:
https://www.youtube.com/watch?v=ODXEYexIqsk
https://www.youtube.com/watch?v=JjPPcR2jqjw
Cad tutorial with caliper measurements and modeling in onshape:
https://www.youtube.com/@CADSessions/videos
Other Onshape video tutorials:
https://www.youtube.com/playlist?list=PL4FdDkwWXT9p6ey_G888zdIinKHHNFGHr
https://www.youtube.com/playlist?list=PLJuwb3xnlvcnqUqOZAJoMLC7VpwHHpx5h
https://www.youtube.com/playlist?list=PLGmYIROty-5ayJwy-Yrb7plI6GglPOomg

SLICING:

Superslicer:
https://github.com/supermerill/SuperSlicer/releases

Prusa Slicer:
https://www.prusa3d.com/page/prusaslicer_424/

Orca slicer:
https://github.com/SoftFever/OrcaSlicer/releases/

Craftware:
https://craftbot.com/craftware

For the old Craftbot+ printer I recommend PrusaSlicer/Super Slicer, but you need special setup, see:
>>> Craftbot+ essentials (includung .ini files for Prusa/Super Slicer):
https://github.com/itzgoldenleonard/Craftbot-Plus-essentials
>>> direct link to printer ini file 4 PrusaSlicer/Superslicer:
https://github.com/itzgoldenleonard/Craftbot-Plus-essentials/tree/master/PrusaSlicer%20config%20files/printer

For the new Flsun V400 I recommend OrcaSlicer with its included settings for the FlsunV400.

FIND MODELS:

Printables (Prusa):
https://www.printables.com

Thingiverse (Ultimaker):
https://www.thingiverse.com/

Grabcad:
https://grabcad.com/library