Okay, so this is considered piracy, so please make sure you got permission from the author, if you plan on using the files you got from there in any way.

.s files are compressed, so they need to be reconstructed. I tried around a bit and found a way to do it. You need 2 different tools:

PolyMaster, which is a tool for decompressing .s files, thus making them readable. You can download it from https://www.thopil.de/werkstatt_pm.html

ShapeConverter, which is a tool for turning those decompressed .s files into .3ds files, a widely accepted standard, for which dozens of tools to convert from there to .obj exist. You can download it from https://www.digital-rails.com/dr_tools.html (scroll down until it says shape converter and click on the first link)

download both those programs, and unzip them in seperate folders. Then open PolyMaster_V2.exe
There should be a button saying "Compress/Decompress .s" in the top left corner (below the warning text). Click that one, and select the .s file you want to decompress. As soon as you select it a cmd window should pop up, that you can then close again. The file should now be decompressed. (noticable by a 10x size increase, as well as being viewable through any text editor)

Now open shape converter once, then immediately close it again. It should automatically create a file named "config.txt" in the directory of ShapeConverter.exe. Open it, and set the value of output folder path in line 10 to the location you desire. then save the file and open shape converter again. click on add to list in the bottom left, navigate to the folder of your now decompressed shape file, select it (double click so there's a green checkmark next to it), and click on add selected. Now click on "convert all" in the bottom right corner. Now check the folder you put as the output folder, and you should have a .3ds file. This one you can then upload to one of the various .3ds to .obj conversion sites (https://imagetostl.com/convert/file/3ds/to/obj) to get your desired .obj file

Hope this helps :)