The translation involved in importing an MS Word file into Pages is done by Pages. The application MS Word has no part to play beyond providing the original file. That file is read and translated in Pages. NO changes are made to the original .docx file.
Similarly, when exporting a Pages file to MS Word file format, the internal pages file is translated to MS Word format by Pages and exported as a .docx file. The original Pages file remains active in Pages, and if it has been saved, remains in the location where you saved it, Note that if you are exporting a file that you previously imported, the new Word file may overwrite the older one if you have not revised the file name since the import.
The usual cautions apply. Translations from MS Word format to Pages format is not perfect,
Any MS Word feature not supported in Pages, or supported in a different manner will not be translated.
The same is true in thr opposite direction.
If you run into problenms, you might check out LibreOffice. This open source and free application offers generally more accurate transltions of MS Office documents to its own format and in the reverse direction than does Pages.
Regards,
Barry