Depends on what you mean by "endgame", because different people have a lot of different opinions on what "endgame" is in Warframe.
If you ask me, the practical "endgame" in Warframe is basically Sorties, Arbitrations, Kuva Floods, and similar content. These missions typically have enemies ranging from level 60 to level 120, and you challenge them to acquire rewards that you can't really get elsewhere. Sorties have a chance to reward precious riven mods, legendary cores, and other valuable trinkets, while Arbitrations are the only means of acquiring Vitus Essence, a special currency.
It's possible to access much more difficult content in Warframe. So-called "endless" mission modes can scale to ridiculous levels, with similarly ridiculous enemies. Some people feel that challenging these modes to their extremes, like hanging around Mot (void survival) for 2 hours, are the real "endgame" in Warframe. However, the rewards aren't any greater than you'll get in the early parts of these modes, so I don't personally think that's all that purposeful or relevant. It's more of a "for fun" thing.
(There are also those who think "fashionframe" is the real endgame, but that's mostly a meme.)
With all of that in mind, considering what I said above (sorties, arbitrations, kuva floods), a typical endgame build looks like the warframe and weapons of your choice kitted-out with fully-ranked mods. Because of the way Warframe works, you can bring almost any equipment to end game content - certainly any Warframe - and contribute. Sure, some gear can turn out better than others, and some gear is better suited to certain mission types than others, but overall, your choice of mods and your own proficiency with the equipment (your skill as a player) are going to be more important than your gear selections.
For primary weapons, you'll typically have Serration, Split Chamber, and either a critical build (including Point Strike and Vital Sense) or a status build (including the 7-drain "60/60" mods, such as Rime Rounds). Some weapons can be built for both criticals and status, called a hybrid build, although doing that typically requires a good Riven Mod. More on Riven Mods in a bit.
For secondary weapons the mod choice is much the same: Hornet Strike, Barrel Diffusion, and then your choice of critical or status, or some combination of both. Critical builds are more popular than status builds for secondary weapons due to the existence of Primed versions of both critical mods: Primed Target Cracker and Primed Pistol Gambit.
Endgame builds for melee weapons diverge a little more than for guns. Critical builds typically make use of a mod called Blood Rush along with combo duration extension mods to ramp their critical chance (and thus damage) to ridiculous levels. Meanwhile, status-focused melee builds are generally based around a rare and expensive mod called Condition Overload, which drastically increases the weapon's damage against enemies based on the number of status effects they're suffering. I haven't seen as many hybrid builds for melee, but they do exist.
Whatever equipment you choose, most of your actual damage against enemies at endgame comes from your weapons, not your warframe. Endgame enemies are typically too strong to effectively deal with them with Warframe powers alone, since most Warframe powers don't scale very well to stronger content. The main exception to this are Warframes that have exalted weapons, although there are a few other exceptions too (e.g. Saryn's infinitely-scaling Spores power).
Regarding Riven Mods:
In theory, the purpose of riven mods is to provide some balance to the game; to make less-used (and thus theoretically, less-powerful) weapons more viable for harder content. In practice there are a number of weapons where equipping a Riven Mod can make the weapon utterly broken mechanically, dealing hundreds of thousands of damage per second, or millions of damage per shot, or similar.
Some players will tell you that Riven Mods are a requirement at endgame. They absolutely are not, for the definition of "endgame" that I've been using. You can complete all of the content in the game without using a Riven Mod. So saying, don't get hung up on whether or not you have a riven mod for a given weapon, and certainly don't pay thousands of platinum for one (unless you're an adult who can afford to spend their money on the game and don't mind feeding the market and supporting DE.) Riven mods can be strong, but they are NOT required to utterly conquer all of the content in Warframe.