iPhone X, XR and XS

They are considered as part of the lineage if iPhones. But each of those was or is a separate model, released at separate times. The iPhone X and XS models are now discontinued (the Xs models replaced with the iPhone 11 Pro and Pro Max).

The iPhone X was just named as it was due to the fact it was released in the 10th year (2017) since the iPhone was first introduced (2007). Note there never was any iPhone 9.

"Considered a line"??? What might that mean?

The iPhone XR is still being sold new by Apple. The iPhone X is not.