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.