When I create an Azure Function App via the portal, it's not possible to select whether an existing Consumption Plan is used or a new one gets created.
However, it is possible to do so via ARM Templates.
Azure Function Apps: Organizing the Azure Function Apps under same Consumption Plan discusses this, but it does suggest no implications.
What are the practical differences between having 1-on-1 mapping between Consumption Plans and Function Apps vs. many Apps in a single Plan?