How are keras tensors connected to layers that create them

In the book "Machine Learning with scikit-learn and Tensorflow" there's a code fragment I can't wrap my head around. Until that chapter, their models were only explicitly using layers - be it in a sequential fashion, or functional. But in the chapter 16, there's this:

import tensorflow_addons as tfa

encoder_inputs = keras.layers.Input(shape=[None], dtype=np.int32)
decoder_inputs = keras.layers.Input(shape=[None], dtype=np.int32)
sequence_lengths = keras.layers.Input(shape=[], dtype=np.int32)

embeddings = keras.layers.Embedding(vocab_size, embed_size)
encoder_embeddings = embeddings(encoder_inputs)
decoder_embeddings = embeddings(decoder_inputs)

encoder = keras.layers.LSTM(512, return_state=True)
encoder_outputs, state_h, state_c = encoder(encoder_embeddings)
encoder_state = [state_h, state_c]

sampler = tfa.seq2seq.sampler.TrainingSampler()

decoder_cell = keras.layers.LSTMCell(512)
output_layer = keras.layers.Dense(vocab_size)
decoder = tfa.seq2seq.basic_decoder.BasicDecoder(decoder_cell, sampler,
                                                 output_layer=output_layer)
final_outputs, final_state, final_sequence_lengths = decoder(
    decoder_embeddings, initial_state=encoder_state,
    sequence_length=sequence_lengths)
Y_proba = tf.nn.softmax(final_outputs.rnn_output)

model = keras.models.Model(
    inputs=[encoder_inputs, decoder_inputs, sequence_lengths],
    outputs=[Y_proba])

And then he just runs the model in a standard way:

model.compile(loss="sparse_categorical_crossentropy", optimizer="adam")

X = np.random.randint(100, size=10*1000).reshape(1000, 10)
Y = np.random.randint(100, size=15*1000).reshape(1000, 15)
X_decoder = np.c_[np.zeros((1000, 1)), Y[:, :-1]]
seq_lengths = np.full([1000], 15)
history = model.fit([X, X_decoder, seq_lengths], Y, epochs=2)

I have trouble understanding the code starting at line 7. The author is creating an Embedding layer which he immediately calls on encoder_inputs and decoder_inputs, then he does basically the same with the LSTM layer that he calls on the previously created encoder_embeddings and tensors returned by this operation are used in the code slightly below. What I don't get here is how are those tensors trained? It looks like he's not using the layers creating them in the model, but if so, then how come the embeddings are learned and the whole model converges?

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