I was aware of this thread, but didn’t want just barge in and waited to see if someone from the staff responds.
The security settings are implemented using the noscript integration.
These are the preferences it sets for each level
// __kSecuritySettings__.
// A table of all prefs bound to the security slider, and the value
// for each security setting. Note that 2-m and 3-m are identical,
// corresponding to the old 2-medium-high setting. We also separately
// bind NoScript settings to the browser.security_level.security_slider
/* eslint-disable */
// prettier-ignore
const kSecuritySettings = {
// Preference name: [0, 1-high 2-m 3-m 4-low]
"javascript.options.ion": [, false, false, false, true ],
"javascript.options.baselinejit": [, false, false, false, true ],
"javascript.options.native_regexp": [, false, false, false, true ],
"mathml.disabled": [, true, true, true, false],
"gfx.font_rendering.graphite.enabled": [, false, false, false, true ],
"gfx.font_rendering.opentype_svg.enabled": [, false, false, false, true ],
"svg.disabled": [, true, false, false, false],
// Expect asmjs to be switched off for all levels. See tor-browser#44687.
// TODO: Stop tracking this preference entirely. See tor-browser#44712.
"javascript.options.asmjs": [, false, false, false, false],
// tor-browser#44234, tor-browser#44242: this interferes with the correct
// functioning of the browser. So, WASM is also handled by NoScript now.
"javascript.options.wasm": [, true, true, true, true ],
};
and these the “capabilities”
// Minimum and maximum capability states as controlled by NoScript.
const max_caps = [
"fetch",
"font",
"frame",
"media",
"object",
"other",
"script",
"wasm",
"webgl",
"noscript",
];
const min_caps = ["frame", "other", "noscript"];
// Untrusted capabilities for [Standard, Safer, Safest] safety levels.
const untrusted_caps = [
max_caps, // standard safety: neither http nor https
["frame", "font", "object", "other", "noscript"], // safer: http
min_caps, // safest: neither http nor https
];
// Default capabilities for [Standard, Safer, Safest] safety levels.
const default_caps = [
max_caps, // standard: both http and https
["fetch", "font", "frame", "object", "other", "script", "noscript"], // safer: https only
min_caps, // safest: both http and https
];
// __noscriptSettings(safetyLevel)__.
// Produces NoScript settings with policy according to
// the safetyLevel which can be:
// 0 = Standard, 1 = Safer, 2 = Safest
//
// At the "Standard" safety level, we leave all sites at
// default with maximal capabilities. Essentially no content
// is blocked.
//
// At "Safer", we set all http sites to untrusted,
// and all https sites to default. Scripts are only permitted
// on https sites. Neither type of site is supposed to allow
// media, but both allow fonts (as we used in legacy NoScript).
//
// At "Safest", all sites are at default with minimal
// capabilities. Most things are blocked.
let noscriptSettings = safetyLevel => ({
__meta: {
name: "updateSettings",
recipientInfo: null,
},
policy: {
DEFAULT: {
capabilities: default_caps[safetyLevel],
temp: false,
},
TRUSTED: {
capabilities: max_caps,
temp: false,
},
UNTRUSTED: {
capabilities: untrusted_caps[safetyLevel],
temp: false,
},
sites: {
trusted: [],
untrusted: [[], ["http:"], []][safetyLevel],
custom: {},
temp: [],
},
enforced: true,
autoAllowTop: false,
},
isTorBrowser: true,
tabId: -1,
});
So you essentially have lists of features that can be used and a classifier which decides which list gets used for what.
Now for the “in practice” part. There is no telemetry in Tor/Mullvad Browser, so nobody from the Tor Project can tell you how the distribution of the various levels is. I would assume that most people are on the default level, followed by the safest level. If the potential gains you get from having some features disabled outweighs the potentially smaller buckets you land in 
I personally see no real reason the safer level exists, since if you actually have to worry about potential 0 day attacks, you should be using safest and not something in between.