I am the new one in ModelBuilder, so have faced some troubles with merging my resulted feature class with my set of input dataset.
Could you please help me to find the mistake?
I think this is what you're after:
Each result feeds into the next input, preconditions (dotted lines) aren't required for a simple operation like this as the script will run left to right.
To add a field use the Add field tool. If you have multiple add fields, which is fairly normal, link them together with the output from one add field feeding into the next add field:
The merge tool can't be used to do what you want; the Append tool is the one that will add existing features to the new feature class.