What is a client in SAP, and why can two clients' data not see each other?

What are the three types of client copy and their transaction codes?

What are the two key differences between a remote client copy (SCC9N) and a local copy?

What is client 000 for, and can application users use it?

What must be done before a client can be accessed by external systems (e.g. BW extraction, S/4HANA↔CRM)?

What is used to purge old client-tool logs and to delete table snapshots, and what is the default retention?

After creating a client with SCC4, does the client physically exist yet? Which table stores its attributes?