The Open Championship format and qualification

The Open Championship qualification was first introduced in 1907, and is the process that a player goes through to qualify for The Open Championship. The Open Championship is the oldest golf competition in the world, and generally regarded as one of the most prestigious. The qualifying structure is designed to reduce more than 2,500 entrants on five continents, to a field size of 156 competing in the tournament.
In the modern era, the main way players qualify is by an exemption due to their performance in major golf tours, major tournaments, or their position in the official world golf rankings. For those who are not exempt, they can qualify by performing well in the Open Qualifying Series of international sanctioned tournaments, or via local qualifying in the UK. 
The format of the final tournament was originally 36 holes played on a single day, but has evolved to the current format of four days of 18 holes, with the lowest scoring golfers cut after the first 36 holes.
Current qualifying process.
Players can qualify one of four ways, an exemption, via the Open Qualifying Series (OQS), via local qualifying, or if the field size has not reached 156, being one of the highest rated players in the official golf rankings that is not already qualified.
Exemption qualifiers.
Around 65% of the field is exempt from qualifying. Not all those eligible for entry will compete (e.g. older previous Open winners, and injured players). 
The current criteria for an exemption is governed by the following rules:
Open Qualifying Series.
Around 22% of the field qualify via the Open Qualifying Series. It was introduced in 2014 and replaced international qualifying events run by the Open. Qualifying is available to the leading players (not otherwise exempt) who finish in the top "n" and ties, in around twelve events run by several international golf tours. The Open Series Qualifying events since 2019 are:
Local and Final qualifying.
Currently 12 players (8% of the field) qualify through final qualifying in the United Kingdom.
Local Qualifying.
Local qualifying begins with 13 events, of 36 holes occurring on a single day just over three weeks before the Open Championship. It is open to those who meet any of the following criteria:
A week later, the best performers then progress to final qualifying, where there are four events of 36 holes occurring on a single day, with three qualifiers from each event.
Final Qualifying.
In addition to the best performers at local qualifying, players who meet the following criteria are able to compete in final qualifying:
"Note: Any qualifier via an amateur performance must still have amateur status"
Alternates.
If the field size has not reached 156, then the highest ranked players not already qualified are offered a spot in the Open.
History of qualification and format.
Qualifying was first introduced from 1907, except 1910–1911, where a 36 hole cut was made instead. 1926 was the first year where there was both a cut and qualifying, and this has been the situation ever since. Qualifying generally happened the day before or shortly before the Open, and everyone was required to qualify until 1962. In 1963 exemptions from qualification began, mainly for past champions, but this has now expanded to cover a multitude of achievements and most of the field now qualify by exemption. In the 1980s, the R&A was concerned that its process was not adequately allowing for players which played on multiple tours, and so actively brought about what is now known as the Official World Golf Ranking to use as part of the exemption process.
In 2004 international qualifying was introduced, which meant that players did not have to travel to the UK to participate in qualifying. This was replaced in 2014 by the Open Qualifying Series, which gave qualifying spots to the best performers in existing global golf events.
The biggest change to the format of the tournament was in 1892 when the tournament was expanded from 36 holes to 72 holes. Another notable change in the rules was making it mandatory to use the "bigger" ball (as was used in America) in 1974.