Keys can be “claimed” by selecting the blue Key link for a product. Some products provide “static keys” that can be used repeatedly without activation limits.
When you search for "Microsoft Visual Studio 6.0 ISO With Key," you are usually looking for one of two editions:
Microsoft Visual Studio 6.0 ISO With Key: A Nostalgic Journey into Legacy Development
Vintage Computing: Hobbyists building period-correct late-90s PC builds often use VS6 as their primary development environment. Conclusion
Visual Studio 6.0 was a comprehensive bundle of specialized development environments: How to get VB 6.0 - Microsoft Q&A