From v15 onwards, adding the assets have been changed and has been moved into the manifest.

Before v15, we were used to inheriting the asset in the XML and adding our custom assets.

But from v15, the asset definition has been moved into the __manfiest__.py of the module as shown below:

'assets': {
    'web.assets_backend': [
        'web/static/src/xml/**/*',
    ],
    'web.assets_common': [
        'web/static/lib/bootstrap/**/*',
        'web/static/src/js/boot.js',
        'web/static/src/js/webclient.js',
    ],
    'web.qunit_suite_tests': [
        'web/static/src/js/webclient_tests.js',
    ],
},

Check assets documentation for more information.