Material's documentation clearly distinguishes between each type of field and its use case:
https://m2.material.io/components/text-fields#filled-text-field
Filled text field Usage
Filled text fields have more visual emphasis than outlined text fields, making them stand out when surrounded by other content and components.
https://m2.material.io/components/text-fields#outlined-text-field
Outlined text field Usage
Outlined text fields have less visual emphasis than filled text fields. When they appear in places like forms, where many text fields are placed together, their reduced emphasis helps simplify the layout.
