Amateur Repeater Linking (repeater-builder.com)
Repeater “linking” can be accomplished through radio linking but more commonly today the Internet is employed we will attempt to explain these options here and offer some options for the repeater “User” (You) to connect through these linking options and allowing you to connect to a “Linked” repeater from anywhere you have internet access.
IRLP Internet Radio Linking Project – Wikipedia
IRLP Summary Page
Asterisk Allstar on the Raspberry Pi 2/3/4 (hamvoip.org)
Building a “Radio-less” Allstar Node (hamvoip.org)

EchoLink
AllStarLink.org
ClearNode – AllStar, EchoLink & DMR – …
How to build your own radioless allstar node
Some very popular “Linked” repeater systems: