Ok. So what about the files on an SD card?
With most SD card adapters you mostly don't work on the SD card level, you put disk images on the SD card and the disk images mount as hard disks on the retro Mac.
I believe there is a way of fetching individual files that are dumped on the SD card, but the normal way is to add file into the disk images, with either a dedicated program for adding files to Mac formatted disk images, or using an emulator.
So, you buy a ZuluSCSI (or similar), then you install Mini vMac, create a 2GB disk image, install Mac System 6.0.8 on it for the Plus, or System 7.1 for the PowerBook (or grab premade images), then use the emulator to drag all the files you want into the disk image. The files should be as you downloaded them, don't decompress them in windows explorer.
I'd suggest decompressing the files in Mini vMac because it will be faster than the retro Macs.
Issues you will have :
- Downloads are in lots of formats and you need numerous programs to uncompress them, including...
Stuffit Expander 4.0.x, Stuffit Expander 5.x, Disk Copy 6.x, Shrink Wrap, MacZip and others.
- You need to set the SD card up correctly, regarding file names and settings, the Plus is picky. See the setup guide for whatever you buy.
- You need to power the SD card adapter externally to use it with a Plus. Unlike on most computers - the Plus was the first SCSI Mac and is... a bit weird. It doesn't provide any power through the SCSI port.
I strongly suggest watching some getting started youtube videos or reading step by step instructions. The questions you're asking are covered in guides to get up and running.