1. Jim says buy at least 3 month duration. What’s wrong with buying the one month ones? I will get the money back sooner and can then redecide what to do with it.
Nothing wrong, just more work for no benefit : )
The yields are usually pretty much identical except when a rate change is expected to be imminent.

2. Is there any friction cost (fees, spread) buying tbills on IBKR?
Commissions are low, if your order is biggish. 0.0012% to 0.0020% of face value? $10 on half a mil?
The problem is that the bid/ask shown is effective only for large trades. When buying a small-to-medium amount your order (probably) won't fill even if you bid at the asking price.
I don't know how much gets their attention, maybe 750k??

3. A lot of options on ibkr when I try to get a quote.. there are strips and many not very clearly understandable names. Which one you guys buy?
Yeah, took me a while to figure it out.
I buy bills.
If you want T-bills, once you have picked the right thing from the pop up menu, the financial instrument should say something like "US-T GOVT Bill Jun 12'25 912797LN5"

4. Say I bought a 1 year bill, and then I decided I need the money (to buy stocks). Do u sell it or get into margin debt?
You could. But you'd be paying half a percent more interest on your margin loan than you'd be earning on the bill.
Personally I would just sell the bill. The bid/ask is tiny, and the value rises as smoothly as you can imagine, so just sell it on the open market. That's why they are considered a substitute for cash. Rather, that's why when people say USD "cash" they really mean T-bills. The market depth is astronomical. Sell a billion without batting an eye.

Jim