Set the basemap to the Imagery basemap.

(Optional) Zoom to one of the areas you want to work offline (Here, McKinley Elementary School). Create a bookmark.

(Optional) Repeat the previous step for each area you want to take offline, making a bookmark for each. (Here, a bookmark is also made for Smiley Elementary School).

Save your map, giving it a name to remind you that it is for an offline project.

Click Map properties in the dark toolbar and click Item details to see the map's item page.

Click the Settings tab and scroll to the Offline settings.

Click Manage Offline Areas.

Go to one of the bookmarks you created (or zoom to the area you want to work offline if you didn't make bookmarks).

Click Create offline area in the pane on the left, click Sketch rectangular map area, and sketch to define the area on the map that you want to take offline.

Set the level of detail you need while offline. For a project on a school campus, consider how detailed the students need to see the basemap. This might be small buildings, and it might be room level.

Since this is a basemap and doesn't often update, expand Packaging schedule and set it to Never.

Name the map area. This is the name you'll see in Survey123 on a mobile device, so it's recommended that you use a combination of the basemap name and the geographic area. (For my first map area in this example, I used McKinley - Imagery.)

Click Save.

Repeat steps 10-14 to add additional map areas for each area you want to work with offline. (For this example, two map areas are made, one for McKinley elementary school and one for Smiley.)

Share the map the same way you did the survey (with your organization or specific groups).

To have offline areas that use another basemap, you'll repeat this process with another map using another basemap. Save your first map with a new name to create the new map, change it to the streets basemap (or whatever other basemap you need), create map areas for the same areas, and share the map.