Hello Vikram.
Regarding 1)
Adding a new system means to connect to a HANA db, not to an ECC or BW system. It can be any HANA server (standalone, used as side-car for another db system or used as primary db for an ECC system, ...).
Regarding 2)
The schemas you see are the schemas on the connected HANA db (consider that you see just that schemas for which you have privileges).
Regarding 3)
New schemas can be created either by the CREATE SCHEMA statement or via a hdbschema repository object.
Regarding 4)
New tables can be created by the CREATE TABLE statement, by a hdbtable repository object or by a Core Data Services entity (hdbdd or hdbcds in HANA >= SPS11).
Best Regards,
Florian