Symptom
You copy a page with modern web parts with the copy option Update links toggled off and notice your links are updated at the destination.
Details
Modern web parts use relative links by default.
A relative URL only contains the path of the targeted resource without the domain.
For example, a link to your about page on your website could be https://yoursite.com/about.
To simplify this link on your site, the URL can be shortened to /about, and any browser will understand to open that link relative to the https://yoursite.com domain.
Because modern web part links are relative by default, no matter how you input them in SharePoint, they will work with the destination even if you specify not to update them in ShareGate Migrate.