Question about moving back to the US and special enrollment periods for individual marketplace insurance

I've been studying outside of the US for a few years. If I move back on September 1, I should be eligible for a special enrollment period… but it's very unclear when a new health plan would start. If I buy a policy this month on the exchange, would it start September 1?* (Or… how much time does it take between buying a policy during special enrollment and that policy taking effect?) Am I able to buy a policy during a special enrollment period online, or do I need to call?

Is there any kind of catastrophic-only coverage I can buy for any gap (I'm older than 30, so not generally eligible for that). I didn't miss dealing with this aspect of life in the US and am very confused.

*It would be for the state of Illinois. It *looks* like Illinois should follow the federal exchange rule of "buy a policy anytime during the month and it starts the first of the next month," so buying after August 15 but before August 31, I could still have a September 1 start date?

ETA: Adding information requested by the moderator. I am 37. State is Illinois. Self-employed, ~$45,000/year pre-tax

submitted by /u/elpislazuli
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