Cannot install .deb or tar.gz files on Debian 11

[frodo8847]

for the deb file I get the error
sudo apt install failed
same with gdebi
archive 'ABDownloadManager_1.5.0_linux_x64.deb' uses unknown
compression for member 'control.tar.zst', giving up

the tar.gz downloads and unpacks with engrampa pkg manager but I cannot
find a top-level README file with instructions what to do from there... there seem to
be no executable files or scripts, only various *.so libraries and a lot of java stuff

both downloads matched on the MD5 sums...

where do I go from here??? thx for any advice.