transferring files between a ppc 6100/66 running 7.5 and a 2015 mpb running catalina

That was one of the first PowerPC-based Macs. So – no USB, no FireWire. We're talking round 9-pin serial ports, SCSI-1, a built-in 1.44 MB floppy drive, and Ethernet. There's an expansion slot, but it is a PDS/Nubus slot, so the chances of finding any sort of USB card for it after all these years are slim and none.

Power Macintosh 6100/60: Technical Specifications - Apple Support

You possibly could transfer files via floppy disks, if you use

• 1.44 MB floppy disks
• FAT formatting
• An external USB floppy drive attached to the 2015 MacBook Pro

If you are trying to save files with resource forks, you'll need to encode them into MacBinary or similar format on the PowerMac 6100 so they can survive a trip through the "data-fork-only" FAT filesystem.

I transferred all user documents from MacPlus ages ago (and saved MacWrite, MacPaint, MacDraw docs as .docx, .jpg etc). AFAIR I used Iomega SCSI ZIP drive and later Iomega USB ZIP drive for that (all that gear still in the basement ready for use or memories...).

...edit: or maybe I used PowerMac 8600/200 SCSI and its ATA or USB or Firewire card for the transfer (that 8600/200 is upstairs ready to dual-boot to System 9 or MacOS X via XPostFacto)... did I already tell you I am a packrat...

Local networking between a System 7.5 Power Macintosh 6100 and a macOS 10.15 Catalina MacBook Pro is not without complication (see, for example, the Networking Interface Chart at http://www.applefool.com/se30/).

One way could perhaps be to use the now free NetPresenz on the 6100 and an appropriate FTP client on the modern Mac (https://happymacs.wordpress.com/2014/07/09/networking-your-classic-macintosh-with-windows-part-3-using-netpresenz-and-fetch/). However, you would have to correctly download/transfer the little NetPresenz application to the PM 6100 in the first place (there are different methods for this, such as a 1.44 MB floppy transfer or by direct download from a plain HTTP site http://www.knubbelmac.de).

If you wish to use a CompactFlash or SD card for transfers to a modern Mac, another approach would be to look for something like a working PowerBook 190/5300/1400/3400 (with a PC Card/PCMCIA slot) as an intermediary (instead of trying to make a SCSI card reader work). Use an inexpensive PC Card adapter for CF or SD. The PowerBook can be networked with the PM 6100 using LocalTalk (a direct MiniDIN-8M to MiniDIN-8M cable should be OK). At a modern Mac (or PC), the CF or SD card can be read via a suitable memory card reader (depending on card formatting and file formats, extra software may or may not be needed).

Additionally I have APS Magneto-optical SCSI drive and AFAIR 128 MB MO disks that work on such old Macs, some of which can also write to SCSI CD drives or to USB.