dendead

Вроде так и получилось)

В MainActivity:

pagerAdapter = new MainPagerAdapter(this);
pagerAdapter.setData(fragment1Container);

В MainPagerAdapter:

public class MainPagerAdapter extends FragmentStateAdapter
{
    private Fragment1Container fragment1Container;
    private Fragment2Container fragment2Container;

    public void setData(Fragment1Container container)
    {
    f   ragment1Container = container;
    }

    public void setData(Fragment2Container container)
    {
        fragment2Container = container;
    }

    @NonNull
    @Override
    public Fragment createFragment(int position)
    {
        switch (position)
        {
            case 0:
                Fragment1 fragment1 = new Fragment1();
        
                Bundle bundle = new Bundle();
                Gson gson = new Gson();
        
                bundle.putString("fragment1Container", gson.toJson(fragment1Container));
                fragment1.setArguments(bundle);
        
                return fragment1;
            case 1:...

В Fragment1:

private Fragment1Container fragment1Container;

if (bundle != null)
{
    Gson gson = new Gson();
    String jsonString = bundle.getString("fragment1Container");
    Type entityType = new TypeToken<Fragment1Container>(){}.getType();
    fragment1Container = gson.fromJson(jsonString , entityType );
}