The current line up is macOS 15, iOS 18, visionOS 2. This implies iOS has been around longer than macOS, which is certainly not the case. It's also confusing for regular folks to keep track of.
I'm sure macOS 26 will keep its nickname, whatever that might be—the current macOS would be known as "macOS 25 Sequoia".
I wish they'd drop the macOS nicknames, as I don't see them used often with the actual version number (only "macOS Sequoia"). So I have no idea if macOS Tomales Bay is older or newer than macOS Salton Sea or macOS Joshua Tree.
I'm sure macOS 26 will keep its nickname, whatever that might be—the current macOS would be known as "macOS 25 Sequoia".