Wifi and cellular
If I get a normal tablet with wifi can I still get a SIM card that has cellular service and use it on that tablet? Or do I need to pitches a tablet with both wifi and cellular?
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If you want integrated Wi-Fi and cellular radios within your iPad, you will need to purchase a model with Wi-Fi and cellular radios. That model will have provision for a SIM, as GSM and LTE networks expect a SIM or eSIM.
If you purchase an iPad model with Wi-Fi radio and no cellular radio, you will have to acquire and use an external hotspot or Mi-Fi or some other such radio for cellular communications, and your iPad will necessarily route your network connection through that.
An iPad with Wi-Fi and no cellular radio will have no provision for a SIM.
Cellular networks use a SIM or eSIM to identify the cellular subscriber.
Wi-Fi does not use nor need a SIM.
A SIM card is akin to “the chip” seen on a credit card or debit card. The SIM is composed of permanent and unchangeable data storage to uniquely identify a cellular subscriber, and some read-write data storage that some phones can use to store contacts info. iPhone can import SIM contacts data, but does not store contacts on a SIM card.
A SIM card is not a radio transceiver, and does not receive nor emit cellular radio signals.