der Panzer
tank
A heavy, armored military combat vehicle equipped with tracks.
Bei der Militärparade fuhren mehrere Panzer durch die Stadt.
Several tanks drove through the city during the military parade.

tank / shell
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tank
A heavy, armored military combat vehicle equipped with tracks.
Bei der Militärparade fuhren mehrere Panzer durch die Stadt.
Several tanks drove through the city during the military parade.

The meaning of 'Panzer' becomes clear from context. If people are talking about the military, it means the combat vehicle. If they are talking about animals, it means the protective shell. In a historical context, it means armor.
Die Armee hat neue Panzer bestellt.
The army has ordered new tanks.
The word component 'Panzer-' is often used in compound words to express that something is especially protected, reinforced, or 'gepanzert' (armored).
Das Auto des Politikers hat Panzerglas.
The politician's car has bulletproof glass.
The expression 'sich in seinen Panzer zurückziehen' (to retreat into one's shell) is often used metaphorically. It means that a person shuts themselves off emotionally and becomes unapproachable.
Nach der Kritik hat er sich komplett in seinen Panzer zurückgezogen.
After the criticism, he completely retreated into his shell.
sign / shield
'das Schild' and 'der Panzer' can both refer to the hard protective covering of an animal. In this area, 'Panzer' is the more common word, especially for turtles, crabs, or similar animals. 'Schild' sounds more specialized here and describes more of a protective hard outer surface. The words can be confused because both have to do with protection. If you are speaking generally about an animal's hard shell, 'Panzer' is usually more natural.

Der Panzer der Schildkröte ist hart und sehr stabil.
The turtle's shell is hard and very strong.
Das Schild der Schildkröte schützt ihren Körper.
The turtle's shell protects its body.