How to add new field in customer_entity table magento 2 using db_schema.xml?

You can add a new column/field in customer_entity table using db_schema.xml under your module etc folder from Magento 2.3 and Upper version.

You just need your table name to add a new field or column for that table.

I have added a new field called Nickname of the customer, For add nickname field in the customer table, you need to only add one row inside the db_schema.xml file.