In the US, “Junior” pilots typically work as flight instructors until they have built up enough time to no longer be junior. 1500 flight hours is the essential requirement to be an airline pilot, and every hour spent giving instruction counts as a flight hour. It’s not the only way, but it’s the most common way. Airlines don’t fund this; pilots have to work their way up to this level themselves.
The 1500 hour rule was instituted by congress at the request of pilots unions not the FAA or any other regulator. Europe only requires 250 hours and has a similar aviation safety track record to the US in the 21st century.
In Europe it’s different.