Tax on normal brokerage account?

I want to start daytrading as a hobby. I know I won’t make much, if any, but it’s fun. It’s basically gambling with extra steps. Let’s say I make a YOLO account for some fun investing. I put in 10k and go all in on stock A. I then lose 5k due to bad investments. I then sell all my shares of stock A and use that 5k to buy stock B. Stock B explodes and now my account is 15k when i sell. I understand short term and long term gains tax. But for this, do I pay taxes on 5k since that’s my total net gain or taxes on 10k since stock B grew 10k. If the latter, can i use my 3k loss tax deduction to make the taxable income 7k? I hope this question makes sense. Thanks in advance!