I believe that all US departures show the 787-8 seat map by default when booked. Toronto shows the A350-900 by default when booked. At least they show it by default whenever I look at it. In practice, the aircraft flown is not what you are shown early on. ANd the airline is free to change whatever seat map is shown at any time.

Out of ORD my research shows that you will get an angled-flat 787-8 about 40% - 50% of the time. The IAD morning departure is likely to get a 777-300 something like 80% - 90% of the time. Toronto will get either a A350-900 or a 777-200LR with a similar percentage. All the other departures get a mix of any of the long haul aircraft that ET operates.

This is based on my research for a trip between RD and LUN departing the end of August 2022 and returning in mid September. I was using my Silver subscription to FlghtRadar24 which shows a history of detailed flight data for the previous 90 days. So I was basically basing this on the ET flight history between North America and Addis in May 2022 through July 2022.

I just checked the aircraft used on ET575 (ORD-ADD) and over the last 90 days this used a B77L (777-200LR) four times, There were no flights between the two cities using an A359, 21 flights used the B789 (787-9, lie flat) and the remainder sere B788 (787-8) aircraft. This last group was a mix of angled fat and lie flat aircraft. It looks like the last group was made up of a light majority of angled flat aircraft (approximately 40% of the flights). But, as always, YMMV.