Self Assessment for Side Hustle + PAYE Job calculations

Im honestly struggling to understand this and hoping someone can explain. I have a full time job which I Salary sacrifice into a work place pension that leaves me in the 40% tax bracket. I also made £15,910 with a side hustle. The tax is fine for the PAYE job i believe. Its all paid, i have my P60. The £15910 from the side hustle, i was told i can be tax efficient with it as i don't need the money. I should put all into a SIPP and I'll get 25% tax relief free automatically + if i do a self assessment I can get 20% on top. I will owe no tax. The money is untaxed though. So really i shouldn't be getting the 25% top up and i shouldn't be getting 20% more on top with a self assessment right? I asked on the reddit and got an other version. If I made £15910 with a side hustle, I shouldn't get any tax relief and I have to pay NI if i put into a sipp. So i should put my side hustle, minus 20% into a sipp. Get 25% tax relief automatically and my sipp will now have £15910. With the 20% i have in cash, i pay that to hmrc as tax owed as they gave me the relief at source automatically within the sipp. Then the only tax left to pay is NI. So with this information i attemtped to fill in my self assessment and got this. None of the above adds up for me. Heres a screenshot it, The bit in blue is math and logics. Green is me understanding the section. Red is my confusion. Wonder if anyone can help? I feel i might be under paying tax with the assessment i linked above

[External link removed - Admin]

Hi

We can reissue your cheque to your UK forwarding address. To allow us to do his, contact us by webchat or phone via Contact HMRC .
Further guidance on tax relief on your pension is also at Tax on your private pension contributions.
For details on how to claim this in your tax return, look at How to complete your tax return for Self Assessment (there are no box numbers on the online version so you are looking at section 4 for pension relief)

Thank you

Ok i'll look at this