For my grandfather, I started with a combination of Chrome Remote Desktop's assistance feature and later Windows Quick Assist. Windows Quick Assist was easier because the helper needs to sign in and type in the one-time code, not the helped.

However, in the end, even walking him through launching Quick Assist was too error-prone, so I ended up installing Chrome Remote Desktop with a Google account that I controlled and enabled remote access, allowing me to connect without requiring him to do anything.