A lot of good advice here. So I'll just add a little from my experience afterwards. Having the other spouse actually
do the website work is very important. Just looking over one's shoulder, it's easy to say "oh yeah, I can do that". And find out later, that one has questions and grinds to a halt.
Some of the processes upon death will depend on state laws, running them all down in advance and documenting is a very good idea.
Everything on AutoPay needs to be found, and what will happen upon death? Who's the main on the credit cards used?
Online-only banks, and others, can be called to find out what actions will be needed. Most everywhere has a "Life Event, Estate, Trust, etc" department that handles these issues. You can be handed off to that department on the phone by the usual first-line people.
Websites of the institutions usually have detailed account actions under the fine print pages of account setup. I have called and verified with the proper department that what I was reading was accurate for me in my state. Sometimes, I couldn't find it, but the proper person on the phone could direct me to the place on the website while I was online, then I could print it off so I'd have it.
Don't make any assumptions. Check each item out. The word "Joint" is too broad. There's JWROS, Joint in Tenant, etc. each is different, and can be affected by state law.
I've been on the other end of what you are doing. I always assumed I would be the first to die, and we made lists for the future. But didn't really realize that there is a whole 'nother level of detail beyond account numbers, PW's, website names.
It's going to take a lot of time to chase it all down, place by place.
After the fact...

I was pretty good on the investment side, as I handled that. The banking, checking, credit cards, utility accounts, safe deposit box, non-health insurances, etc. was DW's world, and I realized too late that there was more to it than the day to day actions when alive.
I know it sounds odd to say, but the Finality of Death is... eternal silence.
I never expected that DW would be the first to die.