AFAIK They have the best track record of upgrading Android smartphones, across all their models. Most of the upgrades they are doing it largely themselves beyond what SoC-vendor provides.
It's not extremely difficult (though Google makes it harder every year), but does require people dedicated to "simply" upgrading. A team of 10 of decent software engineers (juniors are okay as long as they have proper embedded systems education, and they aren't afraid of huge code bases) should be able to handle that. Rock-stars should be able to manage that single-handedly (again assuming they only do that).
That being said, for Fairphone 5, they are using a ""special chip"" for which Qualcomm will provide longer software support than they do for other chips (But I think they still need to extend upgrades themselves after Qualcomm stop). In this case, IMO, it's just a way for Qualcomm to make more money off long support. (The chip probably has some difference compared to standard Qualcomm smartphone SoC, because modern smartphone Qualcomm SoC may suicide itself pretty fast with a removable battery)
It's not extremely difficult (though Google makes it harder every year), but does require people dedicated to "simply" upgrading. A team of 10 of decent software engineers (juniors are okay as long as they have proper embedded systems education, and they aren't afraid of huge code bases) should be able to handle that. Rock-stars should be able to manage that single-handedly (again assuming they only do that).
That being said, for Fairphone 5, they are using a ""special chip"" for which Qualcomm will provide longer software support than they do for other chips (But I think they still need to extend upgrades themselves after Qualcomm stop). In this case, IMO, it's just a way for Qualcomm to make more money off long support. (The chip probably has some difference compared to standard Qualcomm smartphone SoC, because modern smartphone Qualcomm SoC may suicide itself pretty fast with a removable battery)