This work aims to provide a detailed study on Carrier Aggregation (CA) techniques for 5G New Radio (5G NR) networks while elaborating on CA deployment scenarios, CA-enabled 5G networks, and resource management and scheduling techniques. CA empowers the User Equipment (UE) and the network to aggregate carrier frequencies in licensed, unlicensed, or Shared Access (SA) bands of the same or different spectrum bands to boost the achieved data rates. We also analyze the cross-carrier scheduling scheme in CA-enabled 5G networks for Downlink (DL) resource allocation. The requirements, challenges, and opportunities are addressed in the allocation of Resource Blocks (RBs) and Component Carriers (CCs). The study and analysis of various multi-band scheduling techniques are made while keeping in mind that high throughput and reduced power usage needs to be achieved at the UE. Finally, we present CA as the critical enabler to advanced systems while discussing how it meets the demands and holds the potential to support beyond 5G networks. To conclude, we discuss many open issues in resource allocation and scheduling techniques.