How to open Photo library file created by Mac from IPad?

You cannot directly open the library on an iPad. You can sync your photos using icloud so that both devices see the pictures, update automatically, and edits and deletions can be made from either.

Set up and use iCloud Photos - Apple Support

Why an external SSD? Are you very low on storage on your iPad?

You can connect external drives or SD Cards to your iPad. Then browse the content with the Files.app, not with Photos. Connect external storage devices to iPad – Apple Support (UK)

You can export the photos from your library on your Mac to nested folders on an external drive to preserve the albums as folders. Then use the Files.app on your iPad to view them.

The syncing method described by Matti will be much more convenient, but the synced photos will be stored locally on your iPad and need storage. The syncing will reduce the size and resolution, and you will be able to view the photos, but you cannot edit them unless you create a copy and import it into the Photos Library. And you will not be able to delete the synced photos without syncing again. You could be stuck with synced photos taking up too much storage when travelling and away from your Mac.

Or you can see the photos from your iPad, if you connect the iPad to your Mac with a USB cable.

Then you can download the photos from the iPad with Image Capture to a folder on your Mac, or import them with Photos for Mac into a Photos Library (the library that is currently open in Photos).

I don't want to use cloud because my library is over 380gb (too much money for that monthly). Is there any other way I can export my photos from my MacBook Pro into an external SSD HD so I can see my photos in my iPad while traveling?

I prefer to sync from the Mac Photos library via USB to the iPad (the iPad Pro 9.7" 1st gen has its max 256 GB memory but I have to leave out some movie albums because otherwise the library does not fit).

...the sync process is quite slow and might seemingly (or really...) stall. By trial and error I have found the best practice is to keep Mac Photos open in the background during sync. And when updating album content, first omit that album from the sync (and wait for that sync to finish), then add it back because otherwise there might be blank images in the iPad.

I use Google Photos for sharing some photos and movies to other people because despite its flaws I find it better than sharing via Photos (or other similar systems I tested few years ago).

> syncing will reduce the size and resolution

In my setup original quality and resolution images and movies are synced from the Mac Photos library via USB to the iPad (or is there an option to sync in reduced quality?).

All photos I have synced from my Mac to the iPhone, iPad, iPod Touch have always been downsized - Photos created an iPod Photo Cache in the Photos Library to save the optimized, smaller synced versions for the next syncing to speed it up. It has been device specific, how much the synced photos will be reduced. If your iPad has a display with a very high resolution the device specific downsizing may be skipped. I can no longer test this, as I have enabled iCloud Photos on all mobile devices, and then the manual syncing is disabled.

Check out Google Photos

https://www.google.com/photos/about/