I have a question about merge used by Git.

I'm just learning Git. I am posting a question because I have a question in the merge process.

git checkout -b fetch-a

Add README.md File Content via vi Editor

git commit -m "modify readme"

git checkout master

After the above process, I understood that what I worked on the branch of 'fetch-a' did not actually affect the 'master' branch, and I also understood that running the command 'git merge --no-ff fetch-a' on the 'master' branch would merge with the 'master' branch.

Assuming that there is a Java-related source file, if it is merged, the code added from the source code in 'master' is added, and the deleted source part is automatically deleted and merged into one file?