Self Assessment Q&A on 6th December at 15:00 » Self Assessment and Trade Code. » Go to message
That's right Catherine, you do not need a seperate self assessment return, just a seperate self employment section.
Self Assessment Q&A on 6th December at 15:00 » Expenses » Go to message
There is guidance on gov.uk which gives details about the type of expenses which can be claimed, but not a detailed list of everything. I assume you are referring to self employed trader - the details are here https://www.gov.uk/expenses-if-youre-self-employed
Self Assessment Q&A on 6th December at 15:00 » self assessment » Go to message
If you are trading via youtube then yes you will need to register for self assessment. There is more information here on this link to the Business Income Manual https://www.gov.uk/hmrc-internal-manuals/business-income-manual/bim20000
What Catherine has said is correct on this basis.
Self Assessment Q&A on 6th December at 15:00 » Self Assessment and Trade Code. » Go to message
You should register each of your businesses seperately. There may be a different trade class for each one.