Right Cable to Connect Pro-Ject T1 Phono SB Turntable to Era 100

That’s a ground.  It may or may not be needed.  For the actual audio, you would need a stereo RCA male to stereo 3.5 mm male, like this:

https://www.amazon.com/Amazon-Basics-Adapter-Subwoofer-Gold-Plated/dp/B01D5H8JW0/ref=sr_1_1_ffob_sspa?crid=3BDI1U4D9PMF5&dib=eyJ2IjoiMSJ9._AYvJRJ6a--TE19xreCdi-2pdeF_3agpE8_icx3RloxTbGvdqda_33nX5D2RfsdRxuia8bsnon_pQBBg5CQCg_-GtQW1XfBsfcqPesYjfFpcDUKsf6AJyzt1Huq8fvRnr8J3CFN-yAr3O5gLL_2qkrevb6NRGyaemTWObl2kski3VSLT1XejCbF_IckscIzxMf8iLqqXhbQhwvHty8rIIPrE4S_tI0a5zZFC7Q1WDUA.FN0Wpn3Pkc1vP8eMYzNxRZtZOcLHs9F-W5L3Y94Os-E&dib_tag=se&keywords=rca+to+3.5mm&qid=1710942203&sprefix=rca+to+3.5%2Caps%2C80&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1

Thank jgatie! Do you know why one would need ground connection?

The ground connection can reduce external RF noise and hum by providing a path, other than the audio cables for it to follow.

If you want to use it but the other device does not offer a ground connection you can attach the end to a good ground point.

In this setup I would initially ignore the Ground. If you don’t have a hum issue, continue to ignore the Ground.

Thank you for your suggestion. I have now the turntable to the sonos system via the era 100 and it works ok.
But I still have a few observations from which I would love to hear your input:

2. 1. I find that the sound volume is a bit low and that I have to increase the volume almost to maximum.
3. I have observed as well that every time I change the vinyl the sound goes back to play only on the era 100 which is connected to the turntable and stops playing on all the other speakers.

Are these things normal?