You can link your topics to external entities using the slash command /link <type> <key> and /unlink <type> <key>.
For example:
/link card 1234
/link project team-hq
/link group platform
/link topic abcdefg
/link contact joe
/link server database3
Every link has a "type" (what are we linking to), and a unique identifier (key) for that type (i.e. the card id, the topic id, etc).
Similarly, you can unlink a topic using:
/unlink card 1234
/unlink topic abcdefg
When you link a topic, the linked entities show up in the right sidebar
For every link, you'll find an "I" (info) icon that shows you all the topics linked to that entity and a URL
You can also click the link directly to open the entity in an external system for types that support this (i.e. cards, topics, etc)