What do securities work and investment banking do respectively? What is the relationship between them?

The securities work you mentioned is the securities brokerage work.

Securities brokerage is the transaction of securities in the secondary market. Investment banks mainly charge commissions to listed companies. Of course, they can also make bonds and issue additional shares. The income (average income) of investment banks is much larger than that of securities companies. Of course, investment banks will do some self-management in the secondary market. There are many differences between foreign investment banks and Chinese investment banks.

In fact, investment banks also belong to securities companies, only in the investment banking department. IBD