Do offline changes sync automatically later?

Yes. While your phone is offline, the iOS and Android apps keep each change on the device and send them all to your account, in the order you made them, as soon as the connection is back. You don't need to do anything.

Offline mode is a feature of the apps alone, as What is offline mode? explains. The website needs a connection.

The changes the app keeps

Everything the app would normally send to Inoreader is kept while you're offline: reading an article, marking it as read, saving it, tagging it. When the connection returns, the app shows "Your connection to the internet has been restored." and sends the queue, oldest change first, then refreshes your counters. Opening the app once while online is enough for the queue to go out.

On Android the app also sends the queue before any refresh that reads article states from Inoreader, so an article you read offline doesn't turn unread again, and if something is left over, it retries every minute.

The Android app doesn't queue Mark all as read. If you marked a whole section as read while offline, mark it as read again once you're online.

Sections that stay online

Only the sections you made offline are stored on the phone, as How do I make content available offline? describes. While you're offline, any other section says "There is no internet connection" and "To update your article list, you need to be online."

The changes reach your account like any other, and Does Inoreader sync across devices? explains how they get to the website and your other devices.

The queue of offline changes is part of the apps for every account. The offline sections themselves are part of Offline mode, a Pro feature of the iOS and Android apps.