Here is a site that has 'Big Number' library routines written in, and for, BASIC. There Lots of example .bas files that use the library routines as well. It looks like some were written for FreeBasic, but depending what features they use from FreeBasic, they usually are fairly easy to port to QB64.
Thanks to all for your feedback. I suspected something like Explorer 11 was the problem - guess I'll load Chrome or something as I tried Edge when I went to Win10, but just didn't like it. Thanks, again.
When I click on the 'select code' on a discussion page, then paste into my open QB64 V1.3 editor, I get the line numbers displayed in the code box and extra lines between every QB statement. It did not used to work like this. Do I have a some setting wrong. I'm on Win10, but still using Internet Explorer 11 if that makes a difference.