Snowflake external tables are read-only, which means data manipulation language (DML) operations like INSERT, RENAME, or ALTER cannot be performed on them. However, external tables can be used for query and join operations3.
References: [COF-C02] SnowPro Core Certification Exam Study Guide

Snowflake credits are used to pay for the consumption of resources on Snowflake. A Snowflake credit is a unit of measure, and it is consumed only when a customer is using resources, such as when a virtual warehouse is running, the cloud services layer is performing work, or serverless features are used.
https://docs.snowflake.com/en/user-guide/what-are-credits.html

Query pruning in Snowflake is the mechanism used to limit the number of micro-partitions scanned by a query. By analyzing the filters and conditions applied in a query, Snowflake can skip over micro-partitions that do not contain relevant data, thereby reducing the amount of data processed and improving query performance. This technique is particularly effective for large datasets and is a key component of Snowflake's performance optimization features.
References:
* Snowflake Documentation: Query Performance Optimization