Is it actually possible to install Bitcoin full node on a Raspberry Pi 3B+? A Raspberry Pi 3B+ has only 1GB of ram and this official Bitcoin Core page shows that minimum requirement is 2GB ram and 200 GB storage with 100MB/s read and write speed.

But I see a lot of articles where people installing full node on a Raspberry Pi 3 or even older models. Also I see some tutorials where they install full node and run testnet daemon and they attach 500GB+ external HD. Why would they need an external HD when testnet blockchain size is about 8GB.
I have a Bitcoin full node perfectly running on my raspberry pi 3B (of course, without mining).
The biggest constraint in the installation of bitcoin core (though there are also other implementation of Bitcoin protocol) is the initial blockchain download, because your machine need to verify each and every single transaction inside every block from the genesis block up to now. My raspberry with its poor hardware took about 2 and a half months to complete that procedure, but since that it has been running perfectly.
Speaking about the second part of your question, if you run a bitcoin full node on testnet it's probably for self-learning reasons or the will of doing some little tests before turning to mainnet, so it's convenient for you to already get the memory you will need for the real Bitcoin network.