How can I buy more space

If you mean on the phone itself, you have to buy a model with more storage included. It cannot be upgraded.

If you mean iCloud Storage you can subscribe to more.

More information -->

About storage on your device and in iCloud - "Learn the difference between your device and iCloud storage, ways to manage your space, and where to buy more if you run out." Click here --> https://support.apple.com/HT206504

In addition to what Limnos mentioned above, you can also clear some of the space on your device via -

https://www.youtube.com/watch?v=bnhuHSv_WrI