The Immigration Act 1971 came into effect on 1 January 1973. Under section 1(2) of that Act, a person who was ordinarily resident in the UK on 1 January 1973, is automatically deemed to have settled status (indefinite leave to remain) on that date unless they were either exempt from immigration control or already had the right of abode in the UK. A person will continue to hold settled status unless leave has been revoked or lapsed due to absence from the UK.
A person who requires evidence of settled status or who loses their documentation, may apply for confirmation of this status by making a No Time Limit (NTL) application. NTL guidance is published on the Gov.uk website: https://www.gov.uk/transfer-visa/transfer-to-brp
Information about how someone with indefinite leave to remain can become a British citizen is published on the Gov.UK website: https://www.gov.uk/becoming-a-british-citizen.