Hi,
First and most importantly, make sure you have jdk8 installed and your JAVA_HOME points to it. See this thread for more information Java 9 (JDK 9) support by Unity Android
Now for the Android SDK, as you’ve noticed there were some changes in Android SDK manager UI tool. Old sdk manager (which was started by “android.bat”) was deprecated and now the only way to get a graphical UI for managing android sdk is to download Android Studio. After downloading Android Studio you can follow this official documentation Update the IDE and SDK tools | Android Studio | Android Developers to install Android SDK components. If you really don’t want to download Android Studio and feel comfortable working with command line tools, you can use “sdkmanager” command line tool sdkmanager | Android Studio | Android Developers to install required components.