Can you please explain what this should do? I find the code very hard to read. I guess there could be two things mixed together - like getting a tag from relations tags and getting it from the routes[].tags. When is which scenario used?

Maybe then we can find more suitable code.

It always looks for the relations tags and then routes[].tags, so like this:

1. tags.service
2. routes[].tags.service
3. tags.highspeed=yes
4. routes[].tags=highspeed=yes
5. tags.route
6. routes[].tags.route
7. ...

Ok, I understand now. So, the semantics of getTagValue() is in reality get-tag-or-get-tag-on-child-route. I think this function does two things which are unrelated.

More readable code for me would be:

const getValue = (
  key: string,
  tags: Record<string, string>,
) => {
  if (tags[key]) {
    return tags[key];
  }
  return undefined;
};

const getValueOnAnyRoute = (
  key: string,
  routes: WithTags[],
) => {
  const altElement = routes.find(({ tags }) => tags[key]);
  if (altElement) {
    return altElement.tags[key];
  }
  return undefined;
};

const getService = (tags: Record<string, string>, routes: WithTags[]) => {
  const getVal = (key: string) => getValue(key, tags) ?? getValueOnAnyRoute(key, routes);

Do you get the difference? It is subtle, but code like this makes it easier for next reader to get the meaning. They can only read the const getVal line to get the full meaning, without opening the complicated (boilerplate) accessing code.

Of course, this is more like pseudo-code highlight the idea, it could be probably refactored much more.

If you want, you can edit it in some followup.