Classic used to be free for everyone as a browser applet on minecraft.net . It was always commercial, but it was free for most of its history. It was shut down for a few years because of the death of browser plugins and java applets, but then Mojang ported an older-than-the-last-classic-version version to wasm and made it available again at classic.minecraft.net (for free again)

The version on classic.Minecraft.net was deemed written from scratch to vaguely replicate classic last I heard. There were physics quirks, block textures, and entire blocks that weren’t ever in any version of classic, but were in that release.