You can create Node objects via Kubernetes API - these Node objects are just representations for nodes in the cluster, they must also exists e.g. a machine with a Kubelet.
kubeadm join is a tool and command to join a machine to the cluster as a node. This includes many steps including bootstrapping the node using cryptographic certiticates.
Node object in Kubernetes API
You can create Node objects via Kubernetes API - these Node objects are just representations for nodes in the cluster, they must also exists e.g. a machine with a Kubelet.
See Nodes in the Kubernetes documentation.
Join a machine to a cluster with kubeadm join
kubeadm joinis a tool and command to join a machine to the cluster as a node. This includes many steps including bootstrapping the node using cryptographic certiticates.