stl. files

depends on the printer if you need a slicer program to slice the .stl file for a specific printer.

If you are just starting out and not making many adjustments to your stl file it is pretty simple.  You load the file into your slicing program  on your computer, let it do its thing and the file is converted to G-code which is then loaded into your printer.

Marty

I will defer to Alan's experience, but I am not aware that 3D printers can load and print .stl files directly.  Instead what 99% of 3D printing hobbyists do is to load the .stl file into a slicer program.  This program allows the user to tweak settings such as supports, orientation, size, strength, layer height, quality, etc.  You also have the option of using preset defaults which usually work very well and are optimized for specific printers.  Then the slicer program converts the .stl file into g-code which gives the printer the actual step by step instructions it needs to do the print.  This is transferred to the printer via an SD card or Wifi.

Slicer programs appear daunting when you first launch them due to the hundreds of settings that you can tweak.  No worries though, because usually you can simply use the presets and not touch any of the settings.  Popular slicer programs are Cura, Prusa Slicer, Orca, and Bambu Studio.  There are many excellent tutorials on YouTube that walk you through the basics to get you from .stl file to finished print.  For beginners, I would start with the tutorials by  Chris from ItsMeadeMade. He has excellent beginner's guides on Cura, Orca, and Bambu Studio.

Bob

I use Orca or the Creality slicer, I have the Creality K1C printer.  I don't think any 3D printer is ever going to print "directly" from an STL file, all the printers I'm aware of need a sliced file as they print in layers.  I suppose there may be printers that will internally slice the STL file, that seems like a niche application.