I'm going through ekubo basic Japanese lessons and there's a recurring use of the word こと I don't understand, like in this sentence:
そとで、ものをうっていることが多いです
Translated in the video as
People usually sell something outside
My intuition would be that it is used to turn うっている into a noun (something like selling-activity)...
The こと means [場合]{ばあい}, "case, circumstance, occasion, instance" (See definition #②-6-㋑ in デジタル大辞泉).
You can use 「～ことが多い(です)」 to mean 「～場合が多い(です)」, "It is often the case that..." "in many cases..." "often..." "tend to..." etc.
So, your example sentence can be rephrased this way:
[外]{そと}で[物]{もの}を[売]{う}っていることが[多]{おお}いです。
≂ 外で物を売っている[場合]{ばあい}が多いです。
(lit.) Cases where (people) are selling things outside are many.
→ It is often the case that they sell things outdoors.
Examples of ～ことが多いです:
[子供]{こども}は、ピーマンが[嫌]{きら}いなことが多いです。
Kids often dislike green pepper.
[朝夕]{あさゆう}の[電車]{でんしゃ}は、[混]{こ}んでいることが多いです。
Trains are often crowded in the morning and evening.
[不注意]{ふちゅうい}から、[事故]{じこ}が[起]{お}こることが多いです。
Carelessness tends to result in accidents.
Similar use of this こと:
[価格]{かかく}は、[予告]{よこく}なく[変更]{へんこう}することがあります。
Prices may change without prior notice.
バスはときどき、[遅]{おく}れることがあります。
Buses are sometimes late.
It's called nominalization, and yes, it is used to turn verbs (and verb clauses into nouns).
All we need to treat verb clauses as a noun is by attaching a generic noun to the clause: 「こと」（事）
「の」 can also be used as a noun replacement. The difference is 「こと」 is a more general statement while 「の」 is specific to the context of the sentence.
Further reading:
