The half sink stinks!

[Verbena17]

Can anyone suggest why our kitchen half-sink won’t stop smelling - no matter what i do!
i would say the smell is akin to sewage but not sure - might just be awful drains.

It’s been like it regularly since we had the kitchen extended and they put in a new sink and swapped all the drains about and blocked one off (or perhaps they didn’t!).

I put bleach down, I unscrew the plug part with the plug and clean anything away - this never really gets dirty as I do it pretty often).

As soon as I’ve put something like drain cleaner, gel stuff, bleach, really hot water down it, it will only be a matter of minutes before the smell is there again, whenever we either use that sink or the main sink it joins into.

Any plumbing/drain experts know how i can cure it.
Pictures of sink and under sink plumbing attached.
You can see the smaller sink drainage on the top left.

Does it look like there isn’t a proper U Bend?

My middle sink smells every few months. I go under the sink, unscrew the bottom bit as the water sits in it & if it has pieces of food they sit in it, rot/ferment & stink. Clear it out then bicarb & vinegar after before flushing it out.

Thank you!
Ive just seen a similar thing on Reddit where someone said the top trap is too shallow and the overflow pipe can be too long and get clogged up with water and stagnate.