By default WordPress will append “/blog” to the path in Settings > Permalinks for the root site

My Sites > Network Admin > Sites

For the root site, select Edit > Settings

Find “Permalink Structure”

The existing setting will be as set in the site’s normal Settings > Permalinks, but with the initial /blog specified too.

You can alter the /blog to be something else here.

Now go to the sites Settings > Permalinks and save it to cause wordpress to update htaccess