RCS lives in the default messaging app of the phone: there is nothing to install. Apple describes RCS on iPhone as a way to send texts, high-resolution photos and videos with delivery and read receipts and typing indicators when iMessage is not used.
The comparison with WhatsApp is about reach and consistency. RCS availability depends on the device, the carrier and the country, while WhatsApp behaves the same wherever the app is installed. Which one a brand uses mostly depends on which app its customers open every day in each market.
Many brands treat the two as complementary channels. The playbooks are similar (opt-in, approved formats, measured frequency), so the skills transfer. The RCS business messaging guide explains how access works, and these RCS messaging examples show eight messages an online store can send, from a product carousel to a cart reminder.
Kanal sends WhatsApp messages, and SMS and RCS campaigns from the same app: see RCS marketing for Shopify and WhatsApp campaigns.