FWIW, the Spanish Steps were built in 1726, over 200 years after the RC church at the top of the steps was built.
The church, Trinita dei Monti, is a French church, because the church and the land under and surrounding the church are owned by the government of France.
Construction history:
"There was only one way then to get to the completed church and convent, and that was via a dead-end track up the side of the hill on the line of the present Via Gregoriana. Pope Sixtus, however, commissioned a major road-building scheme centred on
Santa Maria Maggiore, and this church was one of the destinations served.
Domenico Fontana was the supervisory engineer.
"In the Middle Ages, the hills of Rome within the ancient walls were depopulated and covered with vineyards, producing pissy wine which was to attract the contempt of the French. The only thoroughfares were narrow, winding country lanes confined by vineyard walls, and there were surprisingly few of these. (A surviving fragment is by the church of
Santa Balbina.)
"Pope Sixtus ordered a new road, initially named
Strada Felice after him (he was baptized as Felice Peretti) which was to run from
Santa Croce in Gerusalemme via
Santa Maria Maggiore to
Santissima Trinità, and a hairpin bend of a junction with the Via Gregoriana. The dead-end terminus of the road was the gate of the
Villa Medici, later to be famous as the centre of French expatriate society in Rome in the 19th century.
"Fontana got into some difficulty surveying the north end of the new road, and had to leave the church at some height above it by excavating the side of the hill. Hence, he provided a monumental staircase which he copied from the
Palazzo Senatorio on the
Campidoglio. This was finished in
1587."
............
"Proposals to create a direct throughfare up the hill in front of the church were mooted [sic, mounted] immediately after completion at the end of the 16th century, and continued in the 17th. Back then, what was here was a wooded hillside. Finally, in 1717,
Francesco de Sanctis was commissioned to lay out the present oversized version of a Baroque garden feature, which immediately became a casual meeting place for city folk and visitors (the contribution of
Specchi is debated). The work was finished in 1725.
-----------
The Steps are an
a posteriori creation, not an
a priori undertaking that is being suggested for the stairs between Summer and Congress.
The obelisk at the top of the steps was put in place in 1789. It was originally in the
Gardens of Sallust, and was a made-to-order Roman copy (in other words, it's not Ancient Egyptian)." [The Gardens of Sallust were constructed in the First Century C.E.]
As it is a French church, the titular bishop (usually a cardinal) is French, and normally presiding over a diocese or archdiocese in France. The administration of the church is by agreement between the Vatican and the government of France.
The above-referenced Villa Medici and the Medici Garden adjacent to the church are also French national property.
The Spanish Steps were given their English name be either Shelley, or John Keats, two English Romantic poets who lived at the foot of the Steps. In Italy, the Steps are known as La Scalinata di Trinita dei Monti; in French, L'escalier de la place d'Espagne.
At the foot of the Steps is the Monaldeschi Palace, the Palazzo di Spagna in Italian, which houses the Spanish embassy to the Vatican.
The Spanish Steps, circa 1757. In the collection of the Metropolitan Museum of Art.