When I take a photo on my phone, it shows up on my iPad, Mac and Apple TV without me having to do anything else.
I can share photo albums and can drop a photo into an album (from my iPhone, Mac or iPad) and they all are synced.
I can buy cloud storage for backups, photos, etc. and have that shared so all family members can use it.
I can use Find my friends to know where my family members are when we are going to meet somewhere.
When I get a text on my phone, I can reply from my iPad (with keyboard) or Mac.
Apps purchased on my iPhone can be shared across family members.
For me, when it comes to “ecosystem” it’s also that it incorporates not just me but other family members that it integrates with.
I’m sure I could do something with Google Photos, but it seems like it would be more difficult managing photos and albums and it’s always an extra app to install and then having to sync.
There’s a lot of little things too, like when I went to set up my iPhone 8, I can restore the backup from my 7 and go through a few screens to set up fingerprint etc and away I go. I’ve never been able to set up a new android device to have it be an exact copy of my last phone. It’s always redownloading apps, setting up accounts again, etc. etc. I can even set up a new iPad using my lPhone backup to so I don’t have to set that stuff up.
So it’s not just about the ecosystem of Apple Services (like photos, messages, etc) but also about how their hardware devices all interact. My Airpods auto connect to my iPhone and iPad for example.