The bars mean absolutely nothing. Let me repeat, NOTHING. One given phone can have 4 bars, another 2 bars, and each may have exactly the same signal strength.
In order to find out whether or not the 12 is getting better reception you have to go into field test mode and look at the DBm.
Do this by going into the dialer and dial *3001#12345#*
Press the call button
In the next screen, select "serving cell info"
Next select LTE
On the next screen look at the entry on the line rsrp0
This will show your receiving signal strength.
Anything below -80db is terrific, anything below -90db is very good, anything below -100db is good, anything below -110db is fair, anything below -120db is marginal and anything over -120db is not good and can possibly drop the call.
If you have both a 12 and an older 11 with SIM cards in both from the same carrier, you can compare the phones side by side. Be aware that the exact placement of the phones and the angle at which they are held or lie can have an affect, so it's a good idea to swap their positions from time to time. Also be aware that either of the phones may be looking at different towers at any given time, so don't just do this, take a look, and come to a conclusion. Give it some time to get a sense of an average reading over several minutes...the more, the better.
The numbers will change periodically, so have patience and look at the results over a period of several minutes to see if one of the phones averages better than the other.
I myself am very curious because I have always preferred iphones over Android except for the signal strength. I have found that iphones routinely do anywhere between 3db to about 7 or 8db worse than Androids.
My wife has an iphone 11 Pro Max and I have compared it to Pixel 3a and Samsung S20 FE 5G, again seeing about 6 or 7db better using either of the Androids. If the iphone 12 is showing to be better than the 11, I'd consider going back to iphone.
BTW, download speeds have very very little to do with reception either and the differences you see have more to do with network congestion at any given time than anything else.