Driven by recent political history, a key tenet of modern German democracy is openness. Indeed. the design of the refurbishment of the Reichstag building allows for views down into the debating chamber of the Bundestag.
Originally built in 1894 the Reichstag was burnt down in 1933 and remained largely a ruin until German reunification in the 1990s. A free visit to the dome can be arranged by booking online in advance.
If you don’t manage to book before you arrive in Berlin, there is a kiosk on the south side of Scheidemannstraße which may have tickets available.
Full details: Visit the Reichstag building