I’ve “rolled” a couple 401ks into a Vanguard account. Just set up a Vanguard traditional IRA (or Fidelity is good too) and follow their instructions. In both my cases my old 401k admin sent me a check and I forwarded it to Vanguard within a certain time frame. If you don’t know what fund to choose just pick “Vanguard Target Retirement XX” for whatever year you turn retirement age (Fidelity has equivalents).
The reason I say Vanguard or Fidelity is because they have rock bottom fees and also they are huge so they’ve worked this out with basically everyone.