1099 income and solo 401k

I have 1099-misc income. I know I can max out 401k (plus catchup) with solo401k. I also know that i can contribute 20% of my 1099 profit to 401k. Question is how this profit is calculated? is it after 1009 amount - all expenses 9computer, travel, cell phone etc)? Which IRS form this profit contribution reported to? I didn't see it on sch C (may be I missed it..)

To open solo 401k do I have to have my onw EIN? or can I use my spouse solo 401k account (or EIN to setup my solo 401k?), as we file taxes jointly?

Any idea when solo 401k account must be opened and contributed to, in order to be valid for 2020?

Thanks