The reigning Formula 1 champion won Sunday's Dutch Grand Prix from pole position, his second successive victory and career 13th, but he had to earn it by passing Mercedes' championship leader Kimi Antonelli after losing the lead at a restart.
The Briton is still 83 points off the lead but whittling away at the deficit.
"I would say that definitely he is now expressing a higher level of assertiveness," Stella told reporters at Zandvoort.
"The manoeuvre to pass Antonelli I think is a really good demonstration of that. He is demonstrating more self-confidence.
"This is interesting and I quite like it, sometimes it is not demonstrated by being the most aggressive. Sometimes you are self-confident when you think like 'you know what, I'm not going to have a fight now because I may lose in this fight. It's a very long race, see you at the end'."
Stella said that meant having the confidence to back off, while having faith in being able to come through in the long run. He pointed out that in Hungary and the Netherlands, Norris had lost the pole advantage but not his confidence.
"I think assertiveness and confidence are probably the two characteristics that he has improved the most from a non-technical, non-operational point of view," added Stella.
Norris has said the new generation of cars are difficult to drive and he has had to adapt his style to accommodate that. He also gave himself plenty of credit for Sunday's win, saying a driver could still make the difference.
Stella agreed the Briton was driving like a champion and constantly improving, a process that started early in 2025 when "something clicked".
"I think this approach has led Lando to continuous improvements," he said. "It's almost like I see Lando stronger and stronger race by race."
Australian teammate Oscar Piastri has meanwhile gone a year since he last won a race, from pole at Zandvoort last August.
Piastri finished sixth on Sunday, 32 seconds behind Norris and hampered by a slow pitstop that cost him a place on a frustrating afternoon.
Stella said the result was not a fair reflection of Piastri's competitiveness through the sprint weekend.