What expenses *actually* reduce my taxable income? | Freelancer

Hey everyone,I am new to this so please bear with me. I am still going through the wiki little by little.

I am taking over tax declaration duties from my wife after she did it last time I was in japan, but she wasn’t really able to explain me what kind of goods I buy and reduce my taxable income.

I am a freelancer currently doing

IllustrationWeb DevelopmentYoutube/gaming videosFlipping some computers/parts

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My question is: what things out of what I buy for work can be used to reduce the taxable income?

For example, I just bought some computer parts, and i was told by my japanese relatives I can add those expenses to reduce my taxes.

Here is where the nuanced/complication comes from though: since I make gaming videos too, will buying a game/console/hardware for that also be useful for that? Say, I buy a ps5 and make videos for youtube (part of the business), can I include it in my business expenses to reduce the tax I have to pay next year?

Thanks in advance, sorry if it’s an obvious question

by thecreatureworkshop