My team is developing a login/register DB system. According to some guides, it's usually used a combination of props (to set states) and a fetch with POST.

login.js

export default class App extends Component {
    state = {
      usuario: '',
      setUsuario: '',
      senha: '',
      setSenha: ''
    }

  async sendForm() {
    let response = await fetch('http://{My NAT IP}:3000/login', {
      method: 'POST',
      headers: {
        Accept: 'application/json',
        'Content-Type': 'application/json'
      },
      body: JSON.stringify({
        usuario: '',
        senha: ''
      })
    })};

    render() {
      return(
        <View style = { styles.container } >
          <Text style={styles.titulo}>
            BOM DENTE
          </Text>

          <Image
            style={styles.logo}
            source={require('../assets/icon.png')}
          />

          <TextInput
            style={styles.input}
            placeholder="Nome de usuário"
            onChangeText={text => this.state.setUsuario}
          />

          <TextInput
            style={styles.input}
            secureTextEntry={true}
            placeholder="Senha"
            onChangeText={text => this.state.setSenha}
          />

          <TouchableOpacity
            style={styles.botao}
            onPress={() => this.sendForm()}
          >
            <Text style={styles.botaoText}>
              LOGIN
            </Text>
          </TouchableOpacity>
        </View >
      )
  }
}

Controller.js

const express = require('express');
const cors = require('cors');
const bodyParser = require('body-parser');
const models = require('./models');

const app = express();
app.use(cors());
app.use(bodyParser.urlencoded({ extended: false }));
app.use(bodyParser.json())

let user = models.Usuario;

app.post('/login', (req, res) => {
    console.log(req.body)
})

let port = process.env.PORT || 3000;
app.listen(port, (req, res) => {
    console.log("Servidor operacional");
})

P.S.: According to this video, expo requires the NAT IP in the fetch instead of localhost.