Please help me how to make such a mechanic?

My guess would be to run a procedure on entity tick update, check if the entity is of type item entity. If it is, check all nearby entities in a cube around it with size of 1. Check each one to see if it is an item entity, and compare the item that it converts to with a set item that you want to check for, such as a moss block, using a variable to keep track of what items have been found. Then, if the variable for the moss block and the item you want are both true, go back through the item entities and despawn the first moss block item entity (setting its variable back to false to track this) and the first other specified item entity. Finally, in the same if block as the despawning, spawn in the new entity you want to spawn.

I am not able to provide a screenshot of the procedure.

You will need to use the global trigger for entity tick updates, the procedure block that gets entity iterator in a cube, the procedure block to convert an item entity into an item stack, a lot of if blocks, the procedure block that checks if an entity is a certain type, and the mob summoning procedure block.

Overall, it is complicated, but it can be done.