Short answers, pulled from the story.
The Menai Strait formed when ice sheets moved from northeast to southwest across Anglesey and Gwynedd, scouring bedrock along the Menai Strait fault system. As ocean levels rose around 10,000 BC at the end of the last ice age, these deep linear hollows flooded to create the present day channel.
Echmarcach mac Ragnaill fought a battle against Viking Guttorm Gunnhildsson at the Menai Strait where Guttorm won by praying to Saint Olaf. A later raid involving Magnus Erlendsson, Earl of Orkney, is recounted in the Orkneyinga Saga as playing an important role in his life.
Differential tides at the two ends cause strong currents that create hazardous areas like the Swellies between the bridges with local whirlpools. The Caernarfon bar features frequently shifting sand banks while the tide flows north-eastwards before reversing direction after hours.
The Menai Strait hosts very rich sponges and reaches depths of 40 metres with currents exceeding 2 knots due to its sheltered position. It contains about 30 plants of the critically endangered Menai Whitebeam which is an extremely rare species of Sorbus found only in this part of North Wales.
The Menai Bridge opened in 1826 designed by Thomas Telford to carry the A5 road connecting London to Holyhead. The Britannia Bridge opened in 1850 as a rail bridge carrying the North Wales Coast Line from Holyhead to Crewe before being adapted to accommodate the A55 dual carriageway between 1970 and 1972.