Apple does not advertise the exact type of storage used. I guess because it is a technical detail that they do not require users to know about, and also because it is inherently difficult to compare as a "spec item".
You can also see this in your question, as you have confused and mixed together various terms - which makes comparison pointless. You list various storage types as "emmc, hfs, ssd, nvme" - which doesn't make any sense.
SSD means "solid state storage", which means storing data without having any moving mechanical parts. This is typically a form of "flash memory", as opposed to traditional hard drives that have rotating platters.
eMMC is a specific standard for SSDs that require them to conform to a specific electric interface and a data transfer protocol. It is usually in the form of a chip that is soldered on to a PCB.
HFS is a file system developed by Apple, which hasn't really seen mainstream usage in many years. A file system is a way to organize bytes on a disk - it doesn't really matter if that disk is an SSD, a hard drive or something entirely different.
NVMe is a protocol specification for accessing storage. The storage is usually a form of SSD, but it could in theory be almost anything. The electrical interface can be many different things (and even non-electric - i.e. optical).
The items above are in four different categories of things. It doesn't make sense to compare them.
Now to the actual, modern iPads made by Apple in the last few years:
You'll find that all of them (and indeed any iPad ever made) has an SSD in some form. I.e. solid state storage.
None of them have eMMC storage.
None of them uses HFS.
None of them uses UFS.
In fact you'll find that Apple in general do not buy off-the-shelf complete drives. Instead they use raw flash storage chips with their own, proprietary storage controller. For the latest iPads such as the iPad Air 5th Gen, this controller is embedded within the main System-On-Chip (SoC).
For your comparison purposes, you can just assume that every iPad made by Apple in the last 5 years or so have the equivalent of NVMe storage. It might not be actual NVMe storage, but it is the comparable of that.
You ask how to search for this type of information. Google is ofcourse always a good place to start, but as you indicate, it does require a technical background to understand any results you get. You can go to places like iFixIt to read about their tear-downs of various iPad models to see how they are made - but again, it requires technical knowledge in order to be able to understand their material.
The best way forward for you I guess is to ask some place like this.