There are already many sites that talk about Unity, so I won't go into too much detail here because there is no point in writing something similar. If you're new to game development or using Unity, you can look it up in books or on a step-up site where you can learn about it.
For the time being, I would like to introduce some sites and books that can be helpful. (I have removed sites that force some kind of registration)
- Sites that describe Unity
- Unity introductory books
- Unity Development Tutorial
About game production fees
At the very least, you need a computer to do game development, so it costs money for physical things. If it is a game for smartphones, it costs money to buy or rent a smartphone to be used for verification.
However, as software, Unity itself is free to obtain and develop tools. In the case of the free version, there are some restrictions, but as long as it is not a game produced by a company, it is possible to publish the game within the free range.
However, where you publish your game is outside Unity's control, so it can cost you money depending on where you publish it.
Restrictions and plans are written on the official website, so please refer to them.