HI,
Is there a website where I can download MAC operating system(any version) for free?

No... not legally anyway

Thank you so much.

I wish to buy an used MAC installed system to learn about MAc operating system and about installing it. I also wish to learn about the hardware components used how it differs from Intel Pcs. Can you help?

Post an ad on craigslist.org if your looking to buy a used mac. Google the operating system the computer has once you have one picked out.

You can download the Mac OS through 7.5.3 for free, and legally. Here's a page on Lowendmac where you can do so. They also link to a page where you can sign a petition to get Apple to release OS 8.X for free download.

I recommend Lowendmac as a resource if you're going to be working with older Macs.

Here's an index of articles on Mac hardware - don't know how up to date it is, though.