ee.ImageCollection.merge

// Sentinel-2 surface reflectance image collection.
var ic = ee.ImageCollection('COPERNICUS/S2_SR');

// Filter the images to those that intersect Mount Shasta for 3 months
// in 2019 and 2021 (two image collections).
var geom = ee.Geometry.Point(-122.196, 41.411);
var ic2018 = ic.filterBounds(geom).filterDate('2019-07-01', '2019-10-01');
print('2018 image collection', ic2018);
var ic2021 = ic.filterBounds(geom).filterDate('2021-07-01', '2021-10-01');
print('2021 image collection', ic2021);

// Merge the two image collections.
var icMerged = ic2018.merge(ic2021);
print('Merged image collection', icMerged);

# Sentinel-2 surface reflectance image collection.
ic = ee.ImageCollection('COPERNICUS/S2_SR')

# Filter the images to those that intersect Mount Shasta for 3 months
# in 2019 and 2021 (two image collections).
geom = ee.Geometry.Point(-122.196, 41.411)
ic2018 = ic.filterBounds(geom).filterDate('2019-07-01', '2019-10-01')
display('2018 image collection:', ic2018)
ic2021 = ic.filterBounds(geom).filterDate('2021-07-01', '2021-10-01')
display('2021 image collection:', ic2021)

# Merge the two image collections.
ic_merged = ic2018.merge(ic2021)
display('Merged image collection:', ic_merged)