To start with, you need really good records.
1. A side gig is a business or a hobby depending on factors identified by the IRS. See here.
https://www.irs.gov/newsroom/heres-how-to-tell-the-difference-between-a-hobby-and-a-business-for-tax...
A business is reported on schedule C. You report gross income and expenses, and pay income tax and self-employment tax on the net profit. You need a separate schedule C for each business. Businesses with substantially similar activities and expenses can be combined. If you do rideshare, deliver meals and groceries that's all basically one business with similar activities and expenses (driving and mileage). But rideshare and housesitting would be separate businesses and be reported on separate schedule Cs.
2. A hobby is reported in one of two ways depending on what you do.
I will discuss buying and selling items (including collectibles) first.
Selling tangible personal property is a capital transaction reported on form 8489 and schedule D. If you sell for more than your cost basis, you have a capital gain. If you sell for less than your basis, you have a capital loss. However, capital losses on personal property are not deductible and do not offset your gains. So if you sell 10 items with a combined loss of $1000 and one item with a gain of $10, you have a $10 capital gain.
You need a spreadsheet listing your items with a description, date purchased, cost when new, date sold, and selling price. If you don't have receipts, make your best guess and hope you aren't audited. Without proof of your cost, the IRS would be allowed to disallow your costs and declare the entire proceeds as taxable income. A listing made at the time of sale is better than nothing, but not as good as receipts.
If perform a service other than selling tangible property, that income is reported as hobby income or "other taxable income" on your tax return. That would include "making" a collectible object (like, making art from found objects). You report the gross income and you can't deduct any expenses (such as tools, supplies, shipping, and so on). If you are performing the activity with a profit motive, it may be a business, even if the sales volume is low. Refer to the multi-factor test above. If a business, you report it on schedule C. You can deduct costs for supplies, tools, listing and selling fees, shipping, and so on.
3. For your graphic design side gig, there is no income threshold. If you file a tax return for any reason, you must report all your income, no matter how small the individual items are. Again, you can either report it on schedule C as a business activity or as hobby "other" income.
