Is it possible to display the UI before a page transition even after the transition?
Also, is it possible to create a UI in a different location and call it from anywhere?
Depending on your use case - have a look in to reusable elements.
These are sort of ‘blocks’ you create once and then add where you’d like them 
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Do you mean iframes? (in order to include external content from another website within your Bubble application) If so - then yes you can use iframes within Bubble.
Reusable elements are not iframes. They are an elements/page that can be reused multiple times throughout your app. Bubble uses them to dynamically build pages.
Sorry, the iframe wasn’t what I was looking for.
I want something that I can create and use anytime I want, like a function in programming.
Reusable elements are what you want.