Where to learn to build or modify custom bit files for my Mesa cards?

Is it possible you are trying to install full ISE rather then webpack?

I have this in my Licence:

"This is license NODELOCKED to HOSTID=ANY;"

That sounds logical, the instructions I followed to install it had me download the full file not the segmented files (or web pack). A 6Gb download surely is not a webpack

I will look into what I have done. I was admittedly lost immediately when there were no "READ ME"s in the download. I don't feel impressed with Xilinx on the instructions front.

I will keep poking at it and see what I can do.

I think the download is the same, IICRC, the choice of flavors is done at install time

The problem is in the naming of the port, Linux now uses the device ID (en0s3 or something similar) where the license manager is looking for the old "Eth0" from back when the ISE was current.

I went into grub and appended "net.ifnames=0 biosdevname=0" to the end of "GRUB_CMDLINE_LINUX:" and rebooted to get it to change the name. The instructions are here:  support.xilinx.com/s/article/60510?language=en_US

I guess it is a common problem if you know exactly how to word a search to look for it. ("xilinx license manager (NIC) ID: 000000000000")
The license manager now shows all the fields as green.

Either way I have "Process 'Generate Programming File' completed successfully" so I am now able to see if I can modify a file and have the outcome repeat 

The more I fight with Linux the more impressed I am at the work you have been doing with it. Good on you!

I had no issues installing webpack (there was no access to the MAC address at all)
Not sure why your install ins node-locked.

I suppose it could be that I just plain failed to install as a webpack. I also downloaded the 89Gb Vivado SFD to install as well instead of the webpack for it. I like the programs to be portable and offline if I can manage it.

I thought I might play with Vivado on the Diligent Cmod A7-35t fpga dev board in parallel with learning the Xilinx ISE for the Mesa's so I was looking at this as a larger package anyway.

I generally lean away from any "as-a-service" web based programs, and I wasn't sure if that was what was implied by a "webpack" installation.

Webpack is just the free version.