There is plenty of software available and much of it is free to download and even to use.
Below is a non exhaustive list of some of the common softwares in use (with download and licence details)
Bluesky plan is free to download and for use with windows based OS.
When downloaded it comes with 2 free credits to use when exporting the 3D STL models and items that you can generate. Further credits can be purchased easily as needed by paypal. (eg. a surgical guide will cost $10 to export ready to print in house)
It is a very powerful software that can be used for:
orthodontic planning and sequential aligner models
surgical guide generation
…and much much more.
Your learning curve is aided by many youtube videos and a fantastic educational part of the website at www.blueskybio.com
Meshmixer is a general 3D editing programme that is free to download for windows or macs.
There are no licence or export fees applicable and its application is only limited by your imagination:
basic model editing functions (eg. repairs, virtual waxups, model basing/ hollowing)
complex editing (eg. bite splints, full arch implant retained temporaries)