Storage on new iMacs is the same as HD size?

You can get up to 1GB or 2GB of storage on the current iMacs. Go to https://www.apple.com/shop/buy-mac/imac

and then click the "Select" button below any of the models there to see how much storage can be configured.

You can also increase storage by adding external storage (either hard disk or solid state drives - SSD).

Another option is to use iCloud (or other cloud service) to reduce the amount of local storage needed.

It sounds like your Mac has a 3 TB Fusion Drive, consisting of

• A 3 TB mechanical hard drive
• A 128 GB solid-state drive

that are logically combined in software to form a single "drive".

New Macs do not come with mechanical hard drives. They all come with solid-state drives. Solid-state drives are faster than mechanical hard drives, but are also generally smaller and more expensive per byte (especially if you're buying them from Apple).

I would suggest getting an internal SSD with at least 512 GB of storage – even if you have to custom-order an iMac, and pay Apple prices for the SSD upgrade, to get it. As for external storage, as long as you don't need Thunderbolt or USB4 40 Gbps speeds, external SSDs are pretty cheap. You could consider getting an iMac with 512 GB – 1 TB of internal storage and augmenting that with a 2 – 4 TB external SSD.

You can get some USB-C (USB 3.1 Gen 2) / NVMe SSDs with 4 TB of storage for less than $250. These won't be as fast as the iMac's internal SSD, but will still, likely, be faster than your Fusion Drive. Or you could get an external 12 TB mechanical hard drive for about $300 – $360. That would be slower and noisier than a SSD – but if you needed that much storage, a hard drive would still have an advantage in terms of capacity and cost per byte.

Note that Apple still uses the name "Macintosh HD" for the startup drive even though no new Macs ship with HDDs!

This is pretty harmless, and you can ignore it, but it does confuse some users.