I think I can do $19,500(employee portion) into individual 401k

Yes

• 25% of my income from the business(

No, assuming self employed. It works out to 20%, as the 20% is deducted from business income (ie. 20% is 25% of (100%-20%)). That's after the deduction of 1/2 of SE payments.

can I also put after tax money in my individual Roth 401k?

Employee contributions can be Roth or Traditional. Employer contributions (the 20%) must be traditional.

Can I also put additional money there, or does the money going into the Roth lower the amount I can put in the individual 401k?

The employee limit is $19.5 (2021; 20.5 for 2022), whether trad, Roth, or a mix.