Since September 2021, it is possible to travel to Canada for travellers again. There are different types of visas for Canada. The most common is the eTA Canada and generally most travellers qualify for this type of travel authorisation. If you are a UK citizen travelling with a UK passport, there is a good chance you qualify for the eTA Canada.
There is not much difference between a visa and the eTA Canada, and usually the only reason to apply for a visa would be that your travel is a bit different than a standard holiday, or you do not meet specific requirements of the eTA Canada. Some of those reasons could be a stay longer than 6 consecutive months, or working or studying in Canada. In these cases, it is not possibly to apply for an eTA Canada.
The Canada eTA requires you to arrive in Canada through their own airports. If you are travelling to Canada via the USA land borders, you do not need an eTA, but you will have to fill in forms at the particular border you enter to Canada. If you travel to Canada via an airport transit in the USA, you need an ESTA to be able to transit through the USA in addition to the eTA you also need a Canada eTA for Transit to other countries.