Infection rates vary by countries, but a lot of it is driven by hotspot outbreaks . And some who seemed to be doing good at first are suddenly getting their hotspot outbreaks at later points.
Anyway, it is more difficult for non-island countries and bigger countries with more global ties.
A temporary closure in the face of a pandemic is somewhat different than closing them based on his rather colourful view of immigrants.
The other "closing of the borders" is about immigration laws. I'm not from the US, but I never understood the anti-Trump position there. Surely, if you have immigration laws, you should try to uphold them. You can criticize the laws and try to change them, but criticizing attempts to uphold the laws makes no sense.