It is likely that Manchester City and Arsenal regularly do well in the Premier League as Manchester City and Arsenal do not have tough opponents in the English Premier League but this is the same thing we do not see in the Champions League. Manchester City is doing well in the Champions League but Arsenal is not doing very well in the Champions League. Arsenal's Champions League journey has come to an end after going from group stage matches to the last round of the Super Sixteen. As the big teams are competing in Champions League, Arsenal and other teams should compete hard so that Arsenal and other teams can finish the tournament in a better Champions League stage.
Arsenal and Manchester City have shown their ability in the Premier League by defending as champions 1 and 2 in the last 2 seasons so I think it's true as you said that they are still very strong in the Premier League their opponents are still below them but don't forget Liverpool because this club will also be a strong competitor in the Premier League last season this club became champion 3 this season maybe there is no significant difference Arsenal and Manchester City will definitely be at the top of the EPL standings.
Talking about the Champions League Man City and Arsenal are different Arsenal does not have a good experience it always ends before the final last season Arsenal was eliminated by Bayern Munich in the quarter finals UCL.
I don't forget Liverpool showed hope in the Premier League last season and at one point it looked like they might finish the season at number one in the points table, behind Manchester City or Liverpool but you must have seen their late game. Their performance towards the end made it look like Liverpool were an unprofessional side and they may have forgotten to win at the end. Liverpool have had a good start in the first two matches this season and they have earned full points in these first two matches but the question here is whether Liverpool can sustain this winning streak till the end of the season. There is no doubt that Manchester City and Arsenal will be consistent, but if Liverpool are discontinuous in the middle of the season, they will not be able to compete with Arsenal or Manchester City in the end.