Hi Angelos and welcome!
I'm not sure I fully understand your question. It looks like you want a popup (e.g. on long press or phrase selection) with results of lookup from an online source (such as Wikipedia). Aard app doesn't do that. A typical use case for Aard is to look up words in the added offline dictionaries, using Aard's interface or by invoking it from a reader app (reader apps like FBReader and MoonReader have specific integrations with dictionary apps including Aard2) or via Android text selection popup menu. When viewing an article in Aard, in wikipedias and wiktionaries article title is also a link that opens the online version of the article. Words and phrases that refer to other Wikipedia articles are typically clickable links, but of course you can also long press any word to bring up standard Android text selection popup and choose an action from there. Some actions - such as Copy or Web search - are built-in, others are provided by apps installed on your device (including Aard 2 itself) - for example "Search Wikipedia" shows up if you have official Wikipedia app installed.
I am familiar with Onyx Boox device - it runs full Android and all above applies to it. It's native built-in book reader (NeoReader) has integration with ONYX dictionary which shows results in a popup, or can be configured to use a 3rd party dictionary app (including Aard 2). Aard 2 doesn't open in a popup though, it's full screen, you just go back to the reader with the back button or gesture when you're done with it.