The Internet Archive has several pages dedicated to older systems include all the Atari ones. Here's how to find and download the files you are interested in.
For the Atari ST, there is one page with a single download (13.9GB) that includes all the Atari categories. In other collections you can download individual zip files for Atari Applications, Educational, Games, Images and more. But on other Internet Archive pages there are individual files for the Atari 2600, 400, 800, 7800, ST, Lynx and Jaquar. All zips are for each type of software (applications, games, etc). Instead of downloading one massive file, you can download just the system/software that you are interested in.
Emulator virtual disk directories on a PC
Once you download the ZIP files you want, it's just a matter of extracting them to a location on your PC's storage. Then depending if you are using an emulator or actual Atari hardware you need to move the files into a format that you can use. If you are using an emulator like STeem for the Atari ST, then you should already have directories on your PC's disk drive to store and access the files. If you have actual Atari hardware, you'll need some utilites to create disk images and then export them to physical disks.
If you actually have the Atari hardware but are struggling to get your software on media that it can use, there's a great resource for modifying Atari hardware and then being able to transfer data to physical disks. It goes into great detail on the USB adapters and other items to connect to your retro equipment. But if you are a hardcore retro geek and you want to get the real experience, this is a great place to get started.
The STeem Atari ST emulator is the leading ST emulator with a mix of outstanding features and compatibility of the ST's software library.
If you are focused on the Atari 16-bit systems and can't afford the crazy prices out there, the STeem emulator is the best choice. Whether you are reliving the old days or just discovering the Atari 16 - bit systems, STeem can operate in color or monochrome mode and can emulate the Atari STF, Mega ST, Atari STE and the Mega STE. The latest release even lets you know if the hardware selected and the verions of the operation system (TOS) are compatible. Finally, check out other Atari ST articles on this site to see the emulator in action and how to configure it for your needs. There are also a couple of valuable ST utilties that you should learn about.
Check out the links below to the Internet Archives, the STeem Atari ST emulator, the Altirra 8-bit emulator and others.
For complete instructions and a walkthrough
watch the video below