Yoshimi

[Impostor]

which leads me back to the start: how can I pick it up for MusE ?

If not already there, put path /usr/lib/lv2 in:
MusE>Global Settings>Plugin Paths>Lv2

And restart MusE. If path's already there, force a plugin rescan (option in same dialog window).
Still can't find the plugin? Maybe it's misrepresented* in the "add synth track" dialog as an effect, so be sure to look under "all" categories.

*due to ubuntu lv2 package possibly being incomplete, if you have ubuntu. Remedy by installing lv2-dev.

[folderol]

I do regularly test Yoshimi with MusE, along with others, so there shouldn't be a problem, but I don't have much experience with ubuntu, so can't comment on that install.

[Largos]

the commands did it ... but only when adding: sudo apt install yoshimi
so: I can start Yoshimi (Version 2.3.2) and in the directory
/usr/lib/lv2/yoshimi.lv2/
i find the files:
manifest.ttl
yoshimi.ttl
yoshimi:lv2.so
which leads me back to the start: how can I pick it up for MusE ?

I'm not sure what you mean by "pick it up" or how new you are to MusE. Imposter gave you instructions for making sure MusE can find the plugin.

There's a beginner tutorial which includes using soft synths here https://muse-sequencer.github.io/docs/beginners.html Unlike most DAW, you need to add a Synth track and then create a midi track to send midi to the Synth.

[topmusik]

I got it
Big thanks to all of you