Mostly humans are replaced at physical labor (although even then not entirely). Human computers and punchcard operators, switchboard and telegraph operators, typesetters/letterpress/linotype operators, draftsmen, photo retouchers, film developers and projectionists, pneumatic tube operators, record-pressing/mastering engineers, the horse-drawn transportation industry, handloom weavers and embroiderers, coopers, wheelwrights, blacksmiths, key cutters, bookkeepers, payroll clerks, proofreaders... This is just a tiny list.
Technology replaces the least efficient parts first, and humans remain to do things that're harder or more expensive to automate safely or reliably. A human who used to build something by hand, transitions to a human who operates a machine to build, then designs or prepares work for the machine, and finally maintains the machine that does it all. For many automated jobs, people now perform maintenance, operations, or design work, that could be done by machine, but we either require or prefer a human do it.
For those remaining jobs, you often still need special skills. But it's no longer a herculean task to perform the work, and the jobs are more specialized and less skilled in general. We still need those jobs, or those automated things simply wouldn't function. Someone has to build them, someone has to maintain them, and someone has to operate them, and each of those requires skill.
So human programmers, systems engineers, designers, architects, operators, etc, will all be very necessary over the next 50 years. You will still need to know the languages, compilers, networks, computers, etc work. You just won't be manually typesetting anymore, or manually weaving the digital cloth. Someone will need to get into the guts of the machine from time to time.