The answer is that, as it stands at the moment and on the deal we have, we would sign the Withdrawal Agreement ('the deal') to end our membership and then enter the transition period.
During the transition period, everything is meant to continue as it is at present (so no to the visas, still free movement of goods and people etc). It is at the end of the transition period that those are all theoretically meant to stop.
If the transition period ends in December 2020, then it should be no change until then and from the end date you would need your visas, there could be tariffs etc. What we need then depends on the deal we negotiate, so it could be complete change or all carrying on as usual.
The two big issues are that the transition period was originally meant to be about 2 years I believe to give time to negotiate a deal with the EU. As we have delayed the Withdrawal Agreement, we are going to end up with less than a year.
Even at 2 years, as a pp said, that does not give enough time for this sort of deal as they can take years, but we are not going to be able to sort it in a year. We do not have the people in place for it (after all it will cover everything we deal with from movement of people, rights, goods, defence, security, air routes and so much more). The EU have already said before that we do not have people with the skill to deal with this. We may have them in the country, but not in the government.
If you think, it has taken around 3 years just to agree to terms for leaving. That was the simple part! That is nothing compared to the scale of the deal we need to agree with the EU and they will have less than a year.