Minecraft expansions

Hi everyone, I'm new to Minecraft. Enjoying the journey so far. Built myself a large base with cave network, nearby village and sea. Built a nether portal, a farm. Mined loads of gold and iron etc.

Trying to get all the basic achievements from the base game.

I noticed additional achievements for a total of 15 expansions.

How do I buy expansions and will they be added to my current base/server or do I need a new server for expansions?

Can I combine all 15 expansions into a single server or do I need 1 serve per expansions?

I'm playing on ps4.