how to purchase space

If you are referring to storage on your iPhone, you will have to replace yours with one that includes more storage than yours currently does. If you are referring to iCloud storage, then the following support article should help:

• Upgrade your iCloud storage plan - Apple Support

If you mean for your iPhone you can’t purchase more storage; it will never have more than it did when you bought it.

If you mean iCloud storage for syncing and backing up, that is explained in the Related Article that you were reading when you posted your message.