web/web_widget_model_viewer/views/assets.xml

<?xml version="1.0" encoding="utf-8" ?>
<odoo>
    <template
        id="web_layout_model_viewer"
        name="Web layout Model viewer"
        inherit_id="web.layout"
    >
        <xpath expr="//meta[@name='viewport']" position="after">
            <script
                type="module"
                src="/web_widget_model_viewer/static/lib/model-viewer.min.js"
            />
            <script
                nomodule="1"
                src="/web_widget_model_viewer/static/lib/model-viewer-legacy.js"
            />
        </xpath>
    </template>
    <template
        id="assets_backend"
        name="web_widget_model_viewer assets"
        inherit_id="web.assets_backend"
    >
        <xpath expr="." position="inside">
            <script
                type="text/javascript"
                src="/web_widget_model_viewer/static/src/js/web_widget_model_viewer.js"
            />
        </xpath>
    </template>
</odoo>