From what I understand, as a solo business owner, I probably should have opened a Solo 401k in the past, but I didn't, and I opened an SEP IRA with Roth (also, shame on Turbo Tax for not including and suggesting doing this when you choose self employed - you shouldn't have to search around in tax tools [jump to is not working this year] to find it!). That aside, if I click maximize on solo 401k it's telling me I can put in $5,000, but if I only click maximize on SEP IRA, it's telling me I can put in $1,260. I thought I should be able to put in the same amount regardless of which account I have? What gives?
The solo 401-K allows more to be contributed. A Sep-IRA is limited to 25% of your compensation or self-employment income. For a solo 401-K, you can contribute up to 100% of your compensation up to $20,500 in 2022 plus an additonal 25% of wages or modified self-employment income. You can read more here: Solo retirement plans
ThomasM125's answer is close, but not entirely accurate. As ThomasM125 indicates, elective deferrals (employee contributions) are permitted to be made to a solo 401(k) but not to a SEP IRA. Both are permitted an employer contribution. However, for a self-employed individual the employer contribution is limited to 20% of net earnings, not 25%. Net earnings are net profit minus the deductible portion of self employment taxes. With a solo 401(k) the employer contribution may be further limited based on net earnings and how much in employee elective deferrals is contributed.
You can see the calculation on Part III of TurboTax's Keogh, SEP and SIMPLE Contribution Worksheet (now that, as of 2/2/2023, the display of Part III has been properly updated for 2022).