This is achieved with the help of something called Autocomplete in discord.js. It's actually pretty simple to do something like this. The first step is to declare an autocomplete command. This can be done in the SlashCommandBuilder by using the option .setAutocomplete(). An example:

const data = new SlashCommandBuilder()
    .setName('command-name')
    .setDescription('command-description')
    .addStringOption(option => {
        option
            .setName('option-name')
            .setDescription('option-description')
            .setAutocomplete(true)
    })

Then, you just have to create an event listener for the interactionCreate event, and then when it fires, check if the interaction was an Autocomplete request and respond with a value. An example:

client.on('interactionCreate', (interaction) => {
    if (!interaction.isAutocomplete()) return
    const enteredValue = interaction.options.getFocused()
    const values = [
        {
          name: 'option-name',
          value: 'option-value',
        }
    ]
    interaction.respond(values)
})

If you want to learn more about the Autocomplete feature, I'd suggest you go here => Autocomplete | discord.js