Strictly speaking, if your bank account is in your sole name, on your death that account is frozen; nobody is allowed to access it. Any instruction you have given to, or written for, the bank lapses on your death. Even if your spouse (or anyone else) is an authorised signatory on your account, the bank will not act on their instructions if they know you are dead, since their authority to act on your behalf died when you did.
The executor/administrator of your estate can access the funds in your account; nobody else. And they have to produce a grant of probate/grant of administration to do so; it takes weeks (at least) to get one.
In practice the banks are a bit more flexible. They have to be; sometimes families would be really, really stuck if a bank account is suddenly and completely frozen. Plus, funerals have to be paid for. As already outlined in this thread, up to a certain limit they will pay out funds to immediate relatives of their deceased customer. But it's discretionary, and you might not want to be depending on it.
The complete opposite is the case with joint accounts. If one of the account-holders dies, the surviving account-holder becomes the sole owner of the account and the funds in it. Nothing is frozen, and they can continue to operate the account in the normal way.
So Dr. Strangelove's advice . . .
married couples should keep funds as much as possible in joint accounts.
. . . is sound. Subject to one important qualification — on death, money (or any other asset) held in a joint account passes automatically to the other account holder; it does not form part of the deceased's estate. So if you and your spouse hold €100k, a bunch of blue-chip shares and heaven knows what else in joint accounts and your will says that your share of them (or part of your share of them) is to go to your nephew, well, it doesn't go to your nephew. It goes to your spouse. It never forms part of your estate and what your will says about it is irrevant.
So, assets should be held in a joint account with your partner
if you want them to go to your partner on your death. Other assets, maybe not so much.