I've been there a few times. Realistically, it's bilingual city and the mother tongue split for Riga itself, is probably 50 - 50.

It does not feel like Ukraine or Belarus. It feels like a mix of any city on the Baltic sea (architecturally) and from the looks of people, also a little bit Scandinavian. Also there is German influence and a fair bit Soviet architecture. It's a quite sympathetic mix really.

All official signs are in Latvian only though. But about half of the people are speaking Russian between themselves.

As you may have heard, Latvia is trying to ignore the fact that close to 40% of the population have Russian as their mother tongue. There is a bit of politics around this question and you'll be surprised how many Russian speakers live there, with seemingly no consideration to their language preference on road signs, commercial signs etc.

They are very tourist oriented in Old Riga and will speak English/Russian and sometimes German as needed , with tourists who are spending money. Baltic people are very good at foreign languages. A lot of people want to practice/show off their skills in English or German and will be just as keen to speak to a foreigner in these languages, as you are to practice Russian!

The market is run by almost only Russian speaking people.
These market vendors were the first Russian speakers who could easily guess that I was Swedish from looks and accent when I spoke Russian. Apparently tons of Swedes turn up at this market and practice scraggly Russian skills there. These people do not speak English, so it's a good place to practice Russian. Good value for money, friendly vendors and a large selection of stuff available - pretty good market.

As for the rest of it; I think the city is split into Latvian and Russian speaking areas, with some overlaps. For example; in the area where my hotel was located, all local people were speaking Russian in shops and between themselves, whereas in the city centre I heard more Latvian spoken. So if you are going to stay there long term, just find out which areas are fully Russian speaking and get a flat there. To some degree you can guess from people's facial features and sometimes their style if they are Russian speakers or Latvian speakers. Probably locals have a good radar for this. I was in Liepaja too, and there it was totally impossible to tell any physical difference between the groups. For some reason it was easier in Riga.

Young Russian speakers in the Baltic states are more or less bilingual though; and 35+ aged Latvians speak Russian to a near native level. Personally I couldn't tell that they were not native speakers.

If you want to be in a Russian speaking environment, in the EU / Baltic states, then go to Daugavpils in Southern Latvia. Rather nice city and it's more or less fully Russian speaking. Not street signs though, but all the people are speaking Russian between themselves. That city feels quite a bit like Belarus which is nearby.

No doubt you can take private Russian tuition while in Riga. If you plan your social life accordingly you can spend your free time with native Russian speakers.