Whilst possible, connecting to your iPhone Cellular hotspot over Bluetooth - instead of making the same connection over WiFi - is largely pointless and detrimental to performance. The WiFi connection has a significantly greater network bandwidth and throughput than Bluetooth.
Each model of iPad is available in two variants - WiFi Only and WiFi & Cellular. Both model variants have WiFi and Bluetooth. The WiFi & Cellular models also have a Cellular radio (for increased flexibility) and GPS functions - neither of which features are available to the WiFi Only devices.
When using an iPhone Cellular hotspot, your iPad can automatically initiate and connect to your iPhone. In summary, the iPad communicates with iPhone over Bluetooth; the iPhone creates an “ad-hoc” WiFi network for its hotspot, to which the iPad connects. In technical terms, the iPhone hotspot uses Bluetooth for “signalling” between the iPad and iPhone - and uses WiFi as the “network bearer” between the devices.
I hope this clarification is helpful in properly informing your choice os iPad.