Full installation, step-by-step, of a Rinnai V94 (similar to V75, V65, V53) on exterior location of house. This model works fine for up to 4 people, 2 bath, 1 kitchen.

Part 1: Modification of existing gas line to extend a 3/4" pipe to the unit (by far the hardest part of this particular installation; skip ahead if not applicable for your situation). (0:43 to 15:31)
Part 2: Install of 3/4" gas line with valve and union per instructions & mounting unit on siding. (15:31 to 25:06)
Part 3: Water supply and water valve kit install. (25:06 to 29:35)
Part 4: Electrical (just final install, not full electrical instruction) (29:35 to 29:45)

Rinnai V Series Tankless Water Heater: eComfort.com or SupplyHouse.com ($600-$1000)
Rinnai Valve Kit: https://amzn.to/3u4yAxn ($105) or SupplyHouse.com
Black Steel Pipe, fittings, nipples: Overbuy at Home Depot or Lowes
Pipe wrenches: Harbor Freight is cheapest
Pex: widely available