It all depends on whether this is a business or a hobby. If this is your business/job, then you need to give receipts and declare the income. It also allows you to deduct business expenses. That is if you are running a business. There can be a lot of advantages to running a business. Gas, equipment, depreciation, advertising, cell phones, postage, lots of things become necessary for the business and thus deductible.
When I was a teen, I cut grass for families around my home. That was the cash and just a hobby. I did not make a living at it. this was just spending money and it never amounted to anything that would require me to even file taxes(below the personal exemption). So there was never a need to give a receipt. But if your green thumb lawn service, you cant do this.
You don't have to explain depositing money in a bank. Many people save and move money between accounts. As long as you don't have regular income without a job. That could become obvious.