All 3 Europeans Leagues compete at the same level. The difference comes solely down to the nations involved.
The Aviva Premiership consists of the best 12 teams in England, with a pyramidal system below (I.e promotion and relegation). In theory a local club team could gain a few promotions from a few leagues down and win, as Exeter Chiefs did last year.
The same is true of the Top 14 in France. Both teams consist of a league based regular season (play all teams home and away) before a knockout playoff system for the top 4-6 teams.
The Guinness Pro 14 is slightly different. This began as the Welsh Scottish League in 1999 featuring the best Welsh clubs and Scottish 'Districts'. In 2001 the 4 Irish Provinces were added to create the 'Celtic League'. In 2003 the Welsh clubs were replaced by 4 regions to create what effectively was a ring fenced competition of franchises. Later 2 Italian teams joined in 2010 followed by two former Super Rugby Franchises in 2017 resulting in the Pro 14 seems today. The competition owners are currently exploring further in the USA, Canada, Germany and Spain.
Hope this helps