Many people might already be knowing this, for example people who use custom keyboard layouts, or people using different keyboard layouts, other than US, which most manufacturers and operating system and desktop environmnets use as default.

Most of my applications use vi like bindings, so pressing shift + ; to get : was a bit annoying, and so i was trying to find a way to swap them, and while doing so, I added most of greek alphabet, makes my technical writing a bit more easier (instead of doing $alpha$, I can do alt + a to get $α$), If you use TeX, then you can use #XeTeX to enter unicode, I use Typst, so I can directly use them, and they are used appropriately in text and equation mode. This also is helpful if your notes system does not have some math mode rendering (like plain markdown, without any katex).

If you are multilingual, but don’t write much in the other language, this approach works well, if you write comparable amount in both languages, then you should consider setting a keyboard shortcut in you DE/WM config to switch the keyboard layouts

The following procedure is linux specific (or maybe other posix systems too, if they use xkb to manage keyboard stuff (this also works on wayland)), but I am sure windows and macos will have something similar (though i have not bothered to check for it, sorry for that). This does not work as a daemon sitting in between, this is using the standard way to modiify keyboard layout.

You can start by making a file like the following, and you can start with /usr/share/X11/xkb/symbols/us (or the base language of your choice, for most european language layouts, base is /usr/share/X11/xkb/symbols/latin). You have to save it in the /usr/share/X11/xkb/symbols/ directory (in my case, my original file is in config dir, and I have symlinked it to /usr/share/X11/xkb/symbols/us-custom). For list of valid symbols (most of unicode, the thing we want is the correct names) please refer /usr/include/X11/keysymdef.h. You can choose what modifier you want to use with include "level3(ralt_switch)" - I am using right alt as the third modifier, so to get the third layer, I go alt + <key>, for 4th layer, mod3 + shift + <key>. Then you can set to use this custom symbol map in your DE/WM config, in my case I have added xkb_layout "us-custom" in my sway config (for input keyboard).

// parent system is US, with a lot of greek sybols appended, and some more
// ; -> :, : -> ; - mostly for vim modes (easier to do : now)
default partial alphanumeric_keys modifier_keys
xkb_symbols "basic" {
    name[Group1]= "English US - Custom"; 
    include "level3(ralt_switch)"

    key <TLDE> {[ grave, asciitilde ]};
    key <AE01> {[ 1, exclam, exclamdown ]};
    key <AE02> {[ 2, at ]};
    key <AE03> {[ 3, numbersign, Greek_pi ]};
    key <AE04> {[ 4, dollar, trademark ]};
    key <AE05> {[ 5, percent ]};
    key <AE06> {[ 6, asciicircum ]};
    key <AE07> {[ 7, ampersand ]};
    key <AE08> {[ 8, asterisk, Greek_PI ]};
    key <AE09> {[ 9, parenleft ]};
    key <AE10> {[ 0, parenright, degree, schwa ]};
    key <AE11> {[ minus, underscore ]};
    key <AE12> {[ equal, plus, plusminus ]};

    key <AD01> {[ q, Q ]};
    key <AD02> {[ w, W ]};
    key <AD03> {[ e, E, Greek_epsilon, eacute ]};
    key <AD04> {[ r, R, Greek_rho ]};
    key <AD05> {[ t, T, Greek_theta, Greek_tau ]};
    key <AD06> {[ y, Y ]};
    key <AD07> {[ u, U ]};
    key <AD08> {[ i, I, integral ]};
    key <AD09> {[ o, O, Greek_omega, Greek_OMEGA ]};
    key <AD10> {[ p, P, Greek_phi, Greek_psi ]};
    key <AD11> {[ bracketleft, braceleft ]};
    key <AD12> {[ bracketright, braceright ]};

    key <AC01> {[ a, A, Greek_alpha, ae ]};
    key <AC02> {[ s, S, Greek_sigma, Greek_SIGMA ]};
    key <AC03> {[ d, D, Greek_delta, Greek_DELTA ]};
    key <AC04> {[ f, F ]};
    key <AC05> {[ g, G, Greek_gamma, Greek_GAMMA ]};
    key <AC06> {[ h, H, Greek_eta ]};
    key <AC07> {[ j, J ]};
    key <AC08> {[ k, K, Greek_kappa ]};
    key <AC09> {[ l, L, Greek_lambda, Greek_LAMBDA ]};
    key <AC10> {[ colon, semicolon ]};
    key <AC11> {[ apostrophe, quotedbl ]};
    key <BKSL> {[ backslash, bar ]};

    key <AB01> {[ z, Z, Greek_zeta ]};
    key <AB02> {[ x, X, Greek_xi, Greek_XI ]};
    key <AB03> {[ c, C, Greek_chi ]};
    key <AB04> {[ v, V ]};
    key <AB05> {[ b, B, Greek_beta ]};
    key <AB06> {[ n, N, Greek_nu, ntilde ]};
    key <AB07> {[ m, M, mu ]};
    key <AB08> {[ comma, less ]};
    key <AB09> {[ period, greater ]};
    key <AB10> {[ slash, question, questiondown ]};
};