Unity game engine is completely free until you start making money with your games. Let's try to understand the different scenarios when you need to pay
Any application made with Unity free version includes the Unity loading screen. If you want to remove that you will need a plus or pro version
If your game or application is making more than $100k/yr and less than $200k/yr then you need to buy the plus subscription.
If your game is making more than $200k/yr then you need to buy the Unity pro subscription.
Both Unity plus and pro are yearly subscriptions. You need to renew them based on your income every year.
Unity Plus Features– Latest version of the Unity Platform– Splash screen customization– Live-Ops analytics– Real-time cloud diagnostics
Unity Pro features– Everything in Unity Plus– Publish to game consoles– Guidance from Partner Advisor– Priority Customer Service– Unity Mars tools for AR/MR– Havok Physics for Unity– Unity Plastic SCM version control
Unity Plus subscription costs $399/yr per seat and the Unity pro subscription cost $2040/yr per seat. You can also pay the subscription on a monthly basis.
Unity Pro users get regular discounts on Unity Assets and also get some paid assets for free along with the plan. These asset reallu compensate the cost of Unity plus.
If your income drops to less than the required value then you don't need to renew the Unity pro or plus license.