What is the difference between the gross and net interest rate?

I am doing freelancing for two different companies and I have several questions. I deliver for them digital files, such as research, PowerPoints or excel files. Do I need to add sales tax to the invoices? When and how do I file taxes? I have never worked before in California and just started working in October last year.

Can anyone share some tips for budding entrepreneurs. What are the do's and dont's for them.

Is commission rebate from the buyer's Realtor to the buyer a taxable income? Transaction was done outside of the escrow and 1099-Misc (Box 7) was issued to the buyer following January. 1) is this taxable income for the buyer and 2) if so, is he responsible for SE taxes as well?

What is the future of a cryptocurrency developer?

If I'm participating in my employer's 401K plan, may I also set up a SEP-IRA for my self-employment income?

How many people actually go in for a Health Insurance? It is observed that most people think it is a waste of money so avoid it. What do you think?

What is a Future Delphi credit scoring and how does it work?