There are several things to do. Here are the basics of lawn care as I have gleaned them from 18 years on lawn care forums:

Watering

Deep and infrequent is the mantra for watering. Deep means 1 inch all at one time. Place some cat food or tuna cans around the yard, and especially in the thin/dead spots. Turn on the sprinkler and time how long it takes to fill all the cans. If the cans in the thin spots never seem to fill while the other cans are overflowing, then you have a sprinkler coverage issue. Once you have that watering time memorized, that will be your watering time from now on. Frequency depends on the daily high air temperature more than any other factor (and there are a bunch). With temps in the 80s, you should be watering once every 2 weeks. Temps in the 70s - once every 3 weeks. Temps lower than 70 should be once every month unless you're frozen.

Mowing

Mow at the mower's highest setting, especially in warmer times of the year.

Fertilizing

Make your first application in late May. Second application in early September. I would strongly suggest using an organic fertilizer like soybean meal or alfalfa pellets (rabbit chow). Those come in 20 kilo bags at the feed store. Use them at a rate of 10 kilos per 100 square meters. The last application should be after the grass stops growing but before the frost hits. This last app should be a straight, fast acting nitrogen like uncoated urea. This is the winterizer and will help the grass to come out of winter dormancy next March/April.

Weeds

Deal with weeds by spot spraying with Weed-b-Gon in mid April and mid September if needed. For those few you have now, pick those out by hand before they become a problem. Spray herbicides go on as a leaf mist, not a soil drench.

The lawn is bumpy because the soil was not prepared right. The surface should have been scraped off to take the high spots off and fill the low spots. If you rototilled first, then you have 3 full years of settling to wait out. It will get bumpier and bumpier until then. That's when you deal with the bumps and holes.

For the dead spots, it would be best to use the same sod from the same supplier. The varieties of KBG seed are numerous. If you use one of the more modern ones and the contractor used something else, you'll have color variations. Having said that, if you want to improve the color of the lawn, say after living with it for a year, then consider getting some of the improved seed varieties, mix them, and scatter in September of 2020. They will take a year or more to really come in with the darker color of green, so you need some patience.

Thanks for all your advice. A few follow up questions:

If I get the seed from the sod farm, should I use that to fill the larger bare area (about 20 by 9 inches) or should I cut out that area and replace it with a piece of sod.

Similarly when I remove the weeds shouldnt I be filling it with seed to avoid the empty space for the weed to regrow ?

Regarding the levelling issue you said that would take 3 years (as he rototilled before installing) - is there anything I can do about it or just leave it as is.

Finally I was wondering what might be a good sprinkler to use. I have a L shaped lawn that's total 1200 sq foot and here's what I currently use:

https://www.lowes.ca/lawn-sprinklers/sun-joe-4500-sq-ft-oscillating-sled-lawn-sprinkler_13518949.html?af=3632&cse=3632&gclsrc=aw.ds&&cm_mmc=--1701537757--66396496956-_-pla-403060845244&gclid=Cj0KCQjwhJrqBRDZARIsALhp1WTyxLbk4qheetfFsIZJB8qE_j34FvgxNJsHyWphGGoNGojq6cProyEaAiX3EALw_wcB

I don't think even in half an hour I get a 1 inch depth in watering with it as it throws the water high up in the air and then droplets fall.

I had to water my sod when I laid it a lot more that once every 2 weeks for the first several weeks. Did you put any fertilizer down under the sod? i used 12-12-12 and it really helped. You dont want anything stronger now. As far as weeds., pull what you can and deal with them latter once it is established. Light rolling cant hurt as well. Just my 2 cents based on my experience here in SW Michigan.