How to maintain a Shimano Nexus gear hub?
I have a bicycle with Shimano hub gears (Nexus 7 speed). I would like to know what type of maintenance it needs, besides the normal tuning.

There are four types of drivetrain maintenance you may want to do on a hub geared bicycle:

Add chain lubricant when the chain squeaks
Re-tension the chain as it wears and the chain slack increases.
Replace the chain and possibly the front and rear sprockets when they are too worn
Re-grease the internal gear hub

Of these, the three first trivial. The problem is the fourth.

I believe it needs some greasing, but I have no idea how to do it.

That's the problem. Few people have an idea how to do it. This is different from derailleur bicycles where the maintenance needed is very simple, although more frequent than on internal geared hub bicycles.
Some instructions are available on the Sheldon Brown web site:

https://www.sheldonbrown.com/nexus-mech.html
https://www.sheldonbrown.com/nexus7.shtml

Internal gear hubs are ideal for people who ride short trips, and are unable to perform maintenance on a bicycle and thus want a bicycle where the service interval is long. People who continue walking and push the bicycle when the rear tire gets a puncture (it's almost impossible to change the rear inner tube on an internal gear hub bicycle unless you consider getting your hands oily acceptable).
If I had an internal gear hub, I would just do chain and sprocket maintenance and ship the hub maintenance. If the hub wears out, it may be a sign that internal gearing might not be acceptable for the use case.