It's good that you found a video for unity.
They're a great help to figure it out.
Everything the video does in the developer console fits.
In the console, you need to create a key to authorize android users and sign it "SHA-1".
You need to create one leaderboard and copy its identifier.
And in the game on Construct3 everything is simple. You conduct a sign-in. After logging in, you can ask for player data and display the leaderboard in the native way. You can create a custom leaderboard using text.
Here's a thread on the forum where I recently tried to help.
construct.net/en/forum/construct-3/how-do-i-8/set-google-play-default-177640