Native Grounds 2nd island in 2nd island zone. You need to load the green crystals in your ships hold and go to the island

Did you click on the plant to feed it? Did you take the shovels and water with you to the island? From your warehouse, tap the water and add it to your boat, same with shovels. Hope this helps.

Click the box in the send pop-up window - you'll see the green check mark - that says something like "Show only who has requested this item" You'll see it only sends to those people and not everyone on your friend list.

I would like a response to this also

You can buy them in the shop with the coins you get for doing the Island Quests and Exploration, or they are a rare drop. The shop has a tab for them.
For the first upgrade I got 3 drops and had to buy the last two with coins I got off the islands as they're different to standard coins but I got most of what I needed exploring the first two chains of islands
Later upgrades look more dependant on refarming those islands to get coins for upgrades though

There's a door with an arrow at the bottom left you click on to leave cave

some things you have to wait until you catch the other quest up to it

You have to keep doing the main quests, it will unlock eventually

I have 5 but did not buy any- guess they just showed up as I advanced in levels...