Spinner spinner = findViewById(R.id.spinner);
    String marca = "";

    //Lista de marcas
    List<String> marcas = new ArrayList<String>();
    marcas.add("Rebook");
    marcas.add("Adidas");
    marcas.add("Nike");

    ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, R.layout.support_simple_spinner_dropdown_item, marcas);
    spinner.setAdapter(adapter);

    spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
                @Override
                public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
                    marca = marcas.get(position);

                }

                @Override
                public void onNothingSelected(AdapterView<?> parent) {
                }
            });

private void guardar() {
    Map<String, Object> info = new HashMap<>();
    info.put("marca", marca);
    FirebaseFirestore db = FirebaseFirestore.getInstance();
    db.collection("nombre_coleccion")
    .add(info)
    .addOnSuccessListener(new OnSuccessListener<DocumentReference>() {
                        @Override
                        public void onSuccess(DocumentReference documentReference) {
                    Toast.makeText(this,"OK",Toast.LENGTH_LONG).show();
    }
                    }).addOnFailureListener(new OnFailureListener() {
                @Override
                public void onFailure(@NonNull Exception e) {
                    Toast.makeText(this,"BAD",Toast.LENGTH_LONG).show();
                }
            });
}