Streamer and radio on Denon tu 260l

Could you please give me some hints on jex proposal

  1. I have a fully functional radio from title. I want to transform this radio also in a streamer.
  2. I want to be able to switch the outputs from radio and a streamer on rpi4.
  3. Please give me a suggestion how to do that.

What you are proposing isn’t feasible with just those two parts.
Your Denon TU-260L is a radio tuner, it tunes AM/FM/LW radio broadcasts and provides line level audio output
A typical Raspberry Pi with DAC hat and Volumio will stream internet radio, play local music files, and stream other internet content such as Spotify, it also provides line level audio output.

So you have two line level audio sources.
You need something to allow switching between the two sources.
You also need something to amplify the signal level to drive speakers (or headphones).
That something would be a stereo integrated amplifier. Denon also make these and something like a Denon PMA-1060 would be contemporaneous with your tuner.
You’d connect the line level output from the Raspberry Pi to the AUX input on the amp, and the tuner to the Tuner input on the amp, it has an input selector to allow switching between all the connected sources.
You’ll also need speakers to complete the audio chain unless you want to use headphones only.