I'd recommend beginning with the prologue or the introductory chapter. It sets the stage and gives you a good foundation for understanding the characters and the world of the manga. Sometimes, there are also special editions or prequels that can enhance your initial experience.
There are several options. Some people find it on dedicated manga websites. Another possibility is to check if your local library has digital access to such manga. Also, some subscription-based manga apps could have it.
Some good places to check for 'Loving Yamada at Lv999' manga are dedicated manga apps such as Comixology or Webtoon. Sometimes, libraries that offer digital manga collections could also have it.
I'm not sure who the specific author of 'Strangers Yamada Novel' is as it's not a widely known title to me. It could be an indie or a relatively unknown work.