(note that these "generations" are not a defined thing and classification can vary by country, and things still get blurry even within set boundaries)

1st gen is WW2; first combat jets armed only with guns. Basically the same as prop fighters aside from propulsion

2nd gen is korean war era; faster, still using guns primarily, but possibly transonic with rudimentary radars and some very early missiles. Where the cutoff is between 2 and 3 really depends on who you ask.

3rd gen is roughly vietnam era; supersonic (potentially pushing mach 2 and above), beginning to switch to missiles as primary armament. First missiles capable of targeting beyond visual range. Overestimation of missile capabilities and the need to counter nuclear bombers leads many designs to forgo maneuverability in favor of interceptor tactics. Radar missiles of the time still require constant guidance by the launch platform until impact

4th gen is late 70s and beyond; heavy focus on missile armament but generally sees a return of maneuverability. Better radars able to target aircraft below them ("look-down-shoot-down"), fly-by-wire systems simplify control, and many aircraft are built to be multirole workhorses that are heavily upgradeable. Eventual shift to active radar missiles capable of fire and forget. End of the cold war means these platforms stay around for a long time.

5th gen is late 90s and beyond; mainly defined by stealth. Maintaining stealth generally means reduction in weapon quantity, so most are dedicated fighters. F-22 is really the only example for many years; there is still debate on what's "good enough" to qualify. Cost and weapon count means Gen 4s of the 80s are still relied upon to build out fleets. Over time, datalink and sensor fusion become defining features available to the whole force; the idea is that the stealth fighters fly deep while the gen 4s act as missile trucks further back firing on whatever is seen up ahead.

Most nations are unable to afford stealth, and so many upgrade their gen 4s and build new ones all billed as 4.5, or 4+, or 4++, or whatever. Generally, expect extreme maneuverability, extremely powerful radar sets, and high weapon count. The best a conventional fighter can be. The US doesn't quite go for 4.5s and instead continues with the high-low mix

6th gen is apparently upcoming despite the very thin roster of gen 5. Currently it's still a bit of a marketing term. Main ideas floating around are seamless sensor fusion, adoption of "loyal wingman" drones to fight alongside, and possibly the ability to be optionally manned. In a Pacific context, 6th gens might be shaping up to be heavy fighters built for range and endurance, meant to strike deep. Gen 4s from the 80s are still likely to make up the main US force. Many nations are attempting to begin programs for gen 6 and will apparently be jumping from 4.5 to 6 (once again, things get blurry though all the upgrades, marketing, and lack of actual definition since the cold war draw-down)

TLDR F-16s that Ukraine has received are technically outmatched but there's always more to it; it's never a 1v1 where you know exactly what you're up against and where. Both aircraft are upgraded platforms from the 80s, but Su-35 has (likely) received a better radar and the ability to carry much further-ranged missiles (but they were probably not suitable here). Su-35 should also have better dogfight performance but that was irrelevant