Does anyone know where I can buy a spare part of this dropout? It’s from my commute bike, but not having a brand… Thanks!
That's a very common design. Here are some links that appear to be compatible.
https://wheelsmfg.com/derailleur-hangers/1-fastener-derailleur-hangers/derailleur-hanger-25.html https://derailleurhanger.com/product/derailleur-hanger-25/ https://www.rearmechhanger.com/product/rear-gear-mech-derailleur-hanger-cc1020/ https://www.jbi.bike/site/product_details.php?part_number=250036
There are various third party derailleur hanger suppliers online (for example https://derailleurhangerstore.co.uk/ - I doubt that's much use to you, but you haven't said where you are). Many of them list the parts by the bike make and model. IF there's any information on the hanger itself, it's not generally very useful. If you don't even know the make/model of your bike, you'll have to look for a visual match, and try to get some information on the threads as well. Some (especially cheap) bikes are sold under several names