Sentinel-2 vs Landsat 8

We have been using only Sentinel 2 data for our site so far.

Landsat 8 has a lower resolution data compared to Sentinel.

Does that mean that vegetation indexes calculated using Landsat would also have a lower resolution?

Can Landsat data be used together with Sentinel data for multi-date L3B processing?

Does Landsat increase quality / accuracy of results compared to using only Sentinel 2 ?

Where could I read more about this?