Forget the patterning and interference for a second.

The process isn’t a one off resist mask and etches you have a series of expose, etch and disposition passes over the material.

The actual feature size can be made much smaller because it can be the product of multiple exposures, etches and depositions.

So let’s say that the smallest feature size you have from a single mask is 200nm by using multiple masks and exposure you basically can build small features which are defined by the overlapping lit or shadow areas of the mask.

These features can also be ethched in a controlled manner to create even finer feature size.

To simplify it even further take make a shadow of an open palm with fingers fully extended so the gap between the fingers is the largest as you can make it and make an outline of the lit areas, now shift your hand slightly to the left or the right and make an outline of the shadowed area and subtract that from the previous outline it's now smaller without you having to change the size of your pattern or the wavelength of the light.

I have no idea really, but the above explanation implies reminds me on the double slit experiments. Also, inference is additive in the amplitude, but phase shifts much smaller than the wavelengths are used in interferometers to measure layer depths.

https://en.wikipedia.org/wiki/Double-slit_experiment

https://en.wikipedia.org/wiki/List_of_types_of_interferomete...

The pattern still is constrained by the wavelength.

What is not constrained by wavelength is deposition and etching of layers, that could be (in theory at least) as thin as you like down to a monolayer.

With multipatterning you can place such small features, but the layout still is constrained by the wavelength.

You’re not taking the resist mask into account, or multiple passes. https://en.m.wikipedia.org/wiki/Multiple_patterning

Not sure how to ELI5 it though...