How can I get address's balance that doesn't belong to my Bitcoin core wallet?
Import addresses in bitcoin core and use listunspent or scantxoutset (scantxoutset doesn't require importing addresses as mentioned by sipa in the comments)

OR

Setup open source block explorers that use bitcoin core and use their API to get the required results:

https://github.com/dgarage/NBXplorer
https://github.com/Blockstream/esplora