How do I get extra storage on my iPad with my iCloud subscription?

It is impossible to increase the amount of RAM or internal storage in your iPad - as this is fixed during manufacture; what you have is fixed for the life of your device. You can, however, subscribe to a higher tier of iCloud storage - but this is not a substitute for internal storage.

Upgrade to iCloud+ - Apple Support

If your internal storage is full, your only option is to better manage the storage that you have - and delete unused Apps or unneeded data. 

These support pages should prove to be helpful:

About storage on your device and in iCloud – Apple Support

What's the difference between device storage and iCloud storage? – Apple Support

Manage your iCloud storage – Apple Support

How to check the storage on your iPhone, iPad and iPod touch – Apple Support

Clear Other storage on your iPhone or iPad - Apple Support

“Offloading” Apps can be a helpful method of creating temporary space for a software update.

https://9to5mac.com/2020/01/21/how-to-offload-apps-in-ios-to-save-space-without-deleting-their-data/

Additional information about creating sufficient space for installation of a software update can be found here:

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

External storage options can often help with managing limited internal storage. Depending upon your iPad model, the iPad will either have a Lightning or USB-C port - and may therefore require either an Adapter or USB hub to connect the USB Storage device. If your iPad has a Lightning port, you’ll need an Apple Lightning to USB3 Camera Adapter:

• Lightning to USB 3 Camera Adapter

https://store.apple.com/uk/xc/product/MK0W2ZM/A

The main thing that iCloud storage can do to free up storage on your iPad is to reduce the space require for photos and videos by using the Optimize Storage option of iCloud Photos. See information in Set up and use iCloud Photos - Apple Support

including the following excerpt:

Turn on Optimize Storage

iCloud Photos keeps all your photos and videos in their original, high-resolution version. To save storage space on your device, you can turn on Optimize Storage.

If you turn on Optimize Storage, iCloud Photos automatically manages the size of your library on your device. Your original photos and videos are stored in iCloud and space-saving versions are kept on your device. Your library is optimized only when you need space, starting with the photos and videos you access least. You can download the original photos and videos over Wi-Fi or cellular when you need them.

If you turn on Download Originals, iCloud Photos keeps your original, full-resolution photos and videos in iCloud and on your device.

On your iPhone, iPad, or Apple Vision Pro

1. Go to Settings > [your name] > iCloud.
2. Tap Photos.
3. Choose Optimize [device] Storage.
4. 
   
5. 

Apps are usually not the biggest hog of memory. Videos generally are, and pictures and music also consume memory. Have you put your photo library in iCloud? If so, it will remove them from your iPad but you will be able to see them when it is connected to the internet.

There are aftermarket devices that will plug into the iPad port that you can store things on, but I’ve never used them myself. You can’t add more hard memory internally to an iPad like you could with older computers with memory slots.

In addition to the good info from TinenTech and FoxFifth, I offer this tip.

Optimize the storage on your current iPhone. You can do this and possibly free up space, by deleting apps, deleting old photos you no longer need, old emails & messages. You can read more about it here: How to check the storage on your iPhone and iPad - Apple Support