You can group the integration solutions into two camps: client- & server-side composition.
With server-rendered applications you can use traditional techniques like server-side includes (SSI), ESI or use a dedicated service (podium, zalando tailor, ara framework) that concatenates your HTML.
For client-side composition AJAX or Web Components are the most often used techniques. There are also library (like single-spa) that provide a custom way of (de-)initialising the individual micro frontends.
In some cases it might even make sense to leverage the good old iframe.