alrighty! so my family all drive fits

I own 2

2010 honda fit Base * as stock as it can get* ,220k m

and a 2015 honda fit *idk what it is but its the knob radio, no sun roof* ,166k m

and my mother owns a 2019 honda Fit, 164k m

Basically fully loaded

i was wondering what i should be doing and checking on the cars, ik the 2015 and 2019 is basically the same since its the same gen but i would love to know if theres anything diffrent to look out for.

as well i always had a fear of changing transmission fluid.
Ive owned and driven multiple cars with 0 oil changes done while owning and 2 of them has hit over 300k miles
so i would like to know on these fits hows that looking like, and how difficult is it for me to do it myself

**Only reason i say i have a fear is because ive heard changing the fluid even though its clean sometimes itll kill the transmission, in a sense that theres no more *risiduals* or whatever and itll start to slip more and blah blah blah*

so after knowing that i just never did or thought its normal to just keep it till something goes
thank you!