We have been using gapless microlenses on imaging sensors for quite a while now. Also, when the pixels shrink, tech advancements in reducing downstream read-noise also mean that very little ISO performance is sacrificed (if any). Certainly, there is a lot more nuance and tech to discuss, but those are the broad strokes.

https://www.photonstophotos.net/Charts/PDR.htm#Canon%20EOS%2...

https://www.photonstophotos.net/Charts/RN_e.htm#Canon%20EOS%...