A portal block set in the End will behave exactly like in the Overworld, calculating coordinates in the Nether with the same 1:8 ratio. They teleport players to the Nether in Java Edition, and to the Overworld in Bedrock Edition.
Latest comment: 5 December 20221 comment1 person in discussion
By using certain methods, nether portal blocks can be obtained as an item: Backporting (at least to 14w21b), and modding. Here is the code for registering nether portal block "item":
1- 1.8-1.8.9:
// Let's register nether portal item!
registerItemBlock(Blocks.portal);
2- 1.9-1.12.2
// Slightly differs from 1.8-1.8.9 method, but only difference is using Blocks.BLOCK key instead of Blocks.block
registerItemBlock(Blocks.PORTAL);
3- 1.13 and onwards
// It does not look like previous method
public static final Item NETHER_PORTAL = registerBlocks(Blocks.NETHER_PORTAL);
I posted a screenshot of this (you can see the nether portal item in the screenshot)