I don't think it is. They usually offer a couple of configurations at different prices, but you can get a BTO machine with just the upgrades you want. The SSD+RAM are tied together in the new M1 iPad.
Ah you're right, i'm the one who confused the two and remembered the weird setup of the iPad, the iMac's Tech Specs page does just state "8GB (configurable to 16GB)" for the entire range (whereas it's quite clear that the "low end" iMac with only 7 GPU cores can only be BTO'd to 1TB SSD, versus 2 for the 8-cores).
