Hi Barath,
welcome to the show!
First of all: TÜV is demotic word, the official term is Hauptuntersuchung (HU).
The TÜV is a technical organization which is (amongst others) certified to carry out the HU, but there are also other organizations as DEKRA, GTÜ,... Many years ago TÜV was the only organization to do the HU, that´s why "TÜV" is a synonym for HU.
The HU is a technical check every two years and it´s mandatory. During this check all parts relevant for safe use of a vehicle are inspected. (No oil leakages, breaks are performing etc.) This is done by visual checks, measurements and a short test ride by the inspector. In addition it is checked if the bike is in the condition as it was homologated. For this purpose it is checked if relevant parts (there is a long list, which parts are considered "relevant") are either original from OEM or are replaced by certified parts from other suppliers. This "certificate" is the ABE (allgemeine Betriebserlaubnis), such parts have a so called "e number" (certificate numer) on the part.
The exhaust is such a relevant part (noise limits became more and more important), thats why it needs to be original or have an ABE (don´t know the term CAT, I assume it means the same). Also modifications on certified parts are not allowed and will be punished. Yes, some people do it and are still alive, but there are better ways to waste money...
There are some suppliers for (legal) exhausts with different sounds. Which one you can find out here in other threads, but I´m afraid you need to exercise your German to find out
Cheers