Is RAM truly plug and play?

Usually yes. Most modern systems will detect new RAM and run it at default speeds without any BIOS changes. The exception is if you’re trying to run RAM faster than the JEDEC standard (like 3200MHz or higher), you’ll need to enable XMP (Intel) or DOCP/EXPO (AMD) in BIOS to get those advertised speeds. If you just swap the sticks and boot, it’ll work, but you might be leaving performance on the table.