How do I add to the order activity timeline?

To add something to the order activity timeline you must create a class that extends \Aero\Admin\Activity:

class OrderFlagsEditActivity extends \Aero\Admin\Activity
{
    protected $view = 'admin::activity.edit-flags';

    protected $key = 'order-flags-activity';

    protected $name = 'Order Flag Edits';

    protected $flags;

    public static function create(\Carbon\Carbon $time, ?\Aero\Admin\Models\Admin $admin, \Illuminate\Support\Collection $flags): self
    {
        return (new self($time))->admin($admin)->flags($flags);
    }

    public function flags(\Illuminate\Support\Collection $flags): self
    {
        $this->flags = $flags;

        return $this;
    }

    public function viewData(): array
    {
        $name = $this->getAdminName();

        return [
            'id' => Str::random(),
            'time' => $this->getTime(),
            'title' => $this->getTitle($name),
            'admin' => $name,
            'changes' => $this->getChanges(),
        ];
    }

    protected function getTitle($adminName): string
    {
        return $adminName ? "{$adminName} edited this order's flags" : "This order's flags were edited";
    }

    protected function getChanges()
    {
        return $this->flags->groupBy('state');
    }
}

And a view to handle rendering the activity:

<div class="w-full [ aero-accordion ]">
    <input type="checkbox" id="{{ $id }}Open" aria-hidden="true">

    <div class="flex items-center">
        <label class="aero-accordion-label aero-accordion-label--inline justify-start flex-1" for="{{ $id }}Open" aria-hidden="true">
            <div class="flex items-center">
                <div class="flex-1">
                    <div class="timeline-title">
                        {{ $title }}
                    </div>
                </div>
            </div>
        </label>

        @include('admin::activity.partials.time', compact('time'))
    </div>

    <div class="visually-hidden">
        <div>
            {{ $title }}
        </div>
    </div>

    <div class="[ aero-accordion-content ]">
        <div class="pl-6">
            @foreach($changes as $state => $stateChanges)
                <div class="py-2">
                    <div class="font-medium mb-2">{{ ucfirst($state) }}</div>

                    @foreach($stateChanges as $change)
                        <div class="flex items-center mt-1">
                            &bull;&nbsp;
                            @include('admin::icons.flag', [
                                'color' => $change->flag->color
                            ])
                            @if($state === \Aero\Admin\Models\FlagHistory::ADDED)
                                <span>&nbsp;{{ $change->flag->name }} @if($change->new_value)({{ $change->new_value }})@endif</span>
                            @elseif($state === \Aero\Admin\Models\FlagHistory::UPDATED)
                                <span>&nbsp;{{ $change->flag->name }} ({{ $change->old_value ?? view('admin::resource-lists.placeholder') }} => {{ $change->new_value }})</span>
                            @else
                                <span>&nbsp;{{ $change->flag->name }} @if($change->old_value)({{ $change->old_value }})@endif</span>
                            @endif
                        </div>
                    @endforeach
                </div>
            @endforeach
        </div>
    </div>
</div>

Finally, you must extend \Aero\Cart\Models\Order to add this activity:

\Aero\Cart\Models\Order::extendActivity(function (\Aero\Cart\Models\Order $order) {
    $order->loadMissing('flagHistoryGroups.admin', 'flagHistoryGroups.histories.flag')->flagHistoryGroups->each(function (\Aero\Admin\Models\FlagHistoryGroup $group) use ($order) {
        $order->addActivity(\Aero\Admin\Activities\OrderFlagsEditActivity::create(
            $group->created_at,
            $group->admin,
            $group->histories
        ));
    }); 
});