How to hook into “register_rest_field” to modify the behavior of a custom field?

I think the most simple solution would be to use a custom filter for this. It could look something like:

$schema = apply_filters('pluginA/register/country', [
    "type" => "string",
    "description" => "The user's country",
    "arg_options" => [
        "sanitize_callback" => function( $value ) {
            return sanitize_text_field( $value );
        },
        "validate_callback" => function( $value ) {
            return is_string( $value );
        },
    ]
]);
register_rest_field( "user", "country", [
    "get_callback" => array( $this, "get_country" ),
    "update_callback" => array( $this, "set_country" ),
    "schema" => $schema,
]);

Now in your other plugin, you simply add a filter and add the required flag or not:

add_filter('pluginA/register/country', function(array $schema): array {
    $schema['required'] = true;
    return $schema;
});

Of course the name of the filter can be freely chosen, it probably makes sense to use some kind of vendor prefix, so it doesn’t clash with filters defined by somebody else.