Dog growls at other dogs when inside yard, but not outside?

We have a really big yard at the front of our house which is fully fenced and has a street next to it. There's quite a few people everyday walking their dogs on that street. Everytime my dog is in the yard, she will bark aggressively and growl at every dog that walks by. If you didn't know her, you'd think she is very suspicious and even aggressive towards dogs.

But now the strange thing is, when she meets dogs on the streets, in training or even when we have visitors and she plays with their dogs in our yard, she is the friendliest dog ever, shows no signs of aggression or anything and is super happy to play with the other dog.

At first I thought she may be protective over her home/territory and that's why she was being "aggressive" towards the other dogs, but literally the second that dog would enter the yard with her, she would be happy and totally friendly.

Maybe she's showing frustration that way because she so desperately wants to play with the dog but can't? I don't know, it's just been confusing me as to why she does that.

also side note she's only 6 months old, idk if that changes anything. but she's been like this ever since we got her from a shelter.