ᐅ Window insulation

Okay, thanks so far... it doesn’t sound very good or rather quite expensive 🙁

link removed by moderation; construction expert

which ones would be the most practical?
Unfortunately, I’m not very wealthy, so blinds will probably have to do...

Construction measures? Are you a tenant or an owner?
If you are a tenant, I would not do anything yourself... At most, you could place draft stoppers in front of the apartment door and on the windowsill. Additionally, you can check whether your landlord has insulated the top floor ceiling and the area toward the basement, and how old your heating system is.
The same applies if you are the owner. Check the measures.

And I repeat: It depends on the overall situation. Please answer the questions that were asked. Then we might be able to help you. A picture often says more than a thousand words.

I assume the OP won’t answer any questions because they were mainly interested in sharing their link :P

Regards