For this example, let's use Brevo.
First, in Brevo, create a new contact list and give it a clear, recognizable name so you can easily distinguish it from your other lists. Then, in your Brevo account settings, generate an API key. Go to Tilda and add it as an integration in the project that contains your contact form. Detailed instructions can be found
here.
Then, on Tilda, open the Settings of the block containing the form and, under "Connected Services," select your Brevo contact list as one of the data receivers.