First of all, you can find some texture mods by Banano_TV on the TM Discord server.
Link to TM Discord:
https://discord.gg/trackmania
Link to the post where he shares 7 different mods, including the one I used in this tutorial:
https://discordapp.com/channels/158349559959912448/159365348510924801/744235605835120660
There's also the cobblestone road mod that htimh made (and hosted by ubm3d). He says people are free to use it. Here's a direct link to the mod:
http://ts.covertstrike.org/zip/Edinburgh.zip
You can only apply mods to an existing map. So you'll need to create the map first. And then...
Paste in one of the mod URLs you get from the Banano_TV post I linked to at the top of this tutorial...
Voila! Your modded map. And since you used a URL, the mod will load for other players.
Just be sure to save your map at the end of the process.
If you want to make your own mod, here's a link to the DefaultTextures for the game:
https://drive.google.com/file/d/1oMFqTvNah2HNLiWf_5G2eC_2g3vlL6HL/view?usp=sharing
It's a good starting point for modders.
Have fun!