For example if I have a collection that has a
field, I'd like to group the options in the select field for a relationship by this
field, like how it is displayed when you allow relationship to multiple collections
This sounds like a customization that would need to be handled from opening the config to support this or using a custom component for this. I think a custom component would be the best move here, once we start work on our component library (specific for payload components, i.e. fields) that we want to open source it should be easier to get up and running with this approach.
Follow along here:https://github.com/payloadcms/payload/discussions/293
Relationship select results custom grouping