The free apps I can think of usually come in the following forms: built-in advertisements, charging the advertiser through user clicks or downloads.

Usually how much does it cost per click and how much does it cost per download.

For example, various free reading software.

In-game recharge and payment, such as various mobile games, are free to download, but if you want a better gaming experience, you can pay to play and have fun.

For example, let go of the Three Kingdoms, and earn copyright fees outside of games by building high-quality brands, such as Plants vs. Zombies and Angry Birds.

These are all well-known brands, such as some children's toys that use Angry Birds icon designs, etc., all of which require copyright fees.

Earn platform users.

Acquire customers through some simple apps, and accumulate customers to sell ads and build a platform.

Some relatively good apps can generally be packaged and sold to large companies or platforms at one time. These apps are generally those that large platforms have no energy to do or have not thought of. A large amount of profit can be earned from a one-time transaction.