open returns a file object. Use with so it closes even if an error happens.

with open("notes.txt", "w",
          encoding="utf-8") as f:
    f.write("first\n")

with open("notes.txt",
          encoding="utf-8") as f:
    for line in f:
        print(line.rstrip("\n"))

• Mode "r" reads and is the default. "w" replaces the file. "a" appends.
• Always pass encoding="utf-8", so text behaves the same everywhere.
• Each line you iterate over keeps its newline. write adds none, so you add it yourself.
• Opening a missing file for reading raises FileNotFoundError.

pathlib.Path wraps a path in an object, with shortcuts for whole-file work:

from pathlib import Path

p = Path("notes.txt")
p.write_text("hi\n", encoding="utf-8")
print(p.read_text(encoding="utf-8"))
print(p.exists(), p.suffix)

Write save_lines(path, lines) writing each string on its own line, replacing any existing file. Write count_nonblank(path) returning how many lines contain something other than whitespace, or 0 if the file does not exist.

The explanation opens here once every check passes.