Practice, practice, practice
I drive by a regional Air Force base and I regularly see them doing touch-and go's with C-17s, C-5s, KC-10s and other commercial models in military gray. Since my drive-bys aren't that long, and I often see them, it's clear they do it all day.
Flying is a perishable skill (meaning you must constantly retrain or you lose it). There's a word for that -- Think of the word "current" as in "up-to-date", then think of how "fluent" becomes "fluency" -- the result is currency, the word for having up-to-date-ness. It also means money, but not here.
Currency is easy enough for a 2-pilot regional-jet crew that lands 4 times in a shift (2 landings per pilot per day). It's a lot harder for a 4-pilot transcontinental jet crew (main, relief) that does 17-hour flights, and lands once a day. (1/4 landing per day).
What's more, not every airport is friendly to touch-and-gos because they simply don't have the runway slots to spare. At JFK, you'd get a mouthful from ole Kennedy Steve. "fuggedaboutit!" Meanwhile some minutes' flying north, at Mirabel, you can have the airport to yourself.
To some extent, simulators are allowed to fill the gap. Otherwise it must be made up with practice flights, touch-and-gos and landings.
A commercial jetliner will do a "go-around" even after touchdown
- if the tower tells them to
- if something is squirreled about their touchdown, like they are too far down the runway and don't have enough left for comfort
- if they actually meant to abort the landing before touchdown, called "go around" and punched up TakeOff/GoAround power, but they touched before the engines fully spooled up -- this might be because of weather, e.g. crosswinds exceeding the pilot's comfort level (google "manchester go-around" for lots of these).
- if they see some sort of traffic contention on the landing runway, e.g. The landing aircraft ahead of them fails to vacate the runway fast enough, or an itchy Cessna at a cross taxiway seems to have missed the stop line
- some other warning, e.g. ATC says "Delta 551, take Bravo (taxiway) to Delta, clear to cross runway 13” and they're landing on 13 right now
It's not for the landing gear. They have indicators in the cockpit for the landing gear. Some have inspection windows, periscopes or cameras so gear can be checked from inside. Otherwise they rely on external inspection: fly by the tower or a chase plane. If they just can't tell, they land anyway, expecting gear to collapse and hoping they won't. How the plane behaves with a collapsed gear is not a surprise; manufacturers design and test so the airplane can land safely with a broken gear.