Although Minecraft certainly has its own physics engine built within Java and Bedrock's codebase, some players might like to see a substantial improvement to the engine.

In Minecraft, all entities and the player's characters have a specific movement speed and size. Some mobs are massive in size, while others are incredibly tiny.

The downloaded file can not be opened as the default launcher of that file is not set to Java. Your antivirus or Microsoft Firewall is blocking the file. Minecraft mods are corrupted.

Importantly, Minecraft Java mods tend to be free, while Bedrock add-ons can be either free or paid downloads in the Minecraft Marketplace.