Flexible Gas Line to Tankless Hot Water Heater

Bear with me, I am not great at relaying info, but I can clarify things if needed! I have been working with a plumber to install a new gas line to the house and just want to make sure that I am informed about some things.

Some context: New gas line is in and we have joints in for each appliance. Nothing has been hooked up as the gas line didn't pass inspection. We currently have a hot water tank that is about to quit and are hoping to swap it out for a tankless water heater. Both appliances have a joint designated for them on the new gas line. We arent having the current plumber install the heater as there have been some (very minor, but albeit) concerns. We have inquired with someone else to do it. The current joint from the main gass line designated for the tankless heater is about 8 feet from the spot we have slotted for the tankless heater.

Questions! How long can a flexible gas line be to a hot water heater? I couldn't Google the answer as the building code info isn't available and in case you could tell, I am very much not a plumber. Is it possible for a plumber to run a flexible gas line from the current spot to the hot water heater or do they need to first install another metal line and then run a flexible gas line to connect the thing?

Please don't chew me out, I will speak with a plumber about it! But I have paid 3 call out fees to 3 different plumbing companies and have been hearing different things.