Google Sheets script - bold words from range in target range

The only way to accomplish this sort of formatting in a spreadsheet is with the use of Class richTextValue and Class richTextValueBuilder all other forms of formatting of this nature apply to entire ranges or cells.

This function will bold all of the selected words in the selected range:

function boldMywords() {
  //const red = SpreadsheetApp.newTextStyle().setForegroundColor('red').build();
  //const org = SpreadsheetApp.newTextStyle().setForegroundColor('orange').build();
  //const blu = SpreadsheetApp.newTextStyle().setForegroundColor('blue').build();
  const bold = SpreadsheetApp.newTextStyle().setBold(true).build();
  //const cA = [red,org,blu];//colors array
  const wordA = ['coffee','phone'];//words array
  const ss = SpreadsheetApp.getActive();
  const sh = ss.getSheetByName('Sheet0');
  const rg = sh.getDataRange();//selected range
  let vs = rg.getDisplayValues();
  
  vs.forEach((r, i) => {
    r.forEach((c, j) => {
      let idxObj = { pA: [] };
      wordA.forEach(w => {
        let idx = c.indexOf(w);
        if (~idx) {
          idxObj[w] = idx;
          idxObj.pA.push(w);
        }
      });
      if (idxObj.pA.length > 0) {
        let cell = sh.getRange(i + 1, j + 1);
        let val = SpreadsheetApp.newRichTextValue().setText(c);
        idxObj.pA.forEach((p,k) => {
          //val.setTextStyle(idxObj[p], idxObj[p] + p.length, cA[k % cA.length]);
          val.setTextStyle(idxObj[p], idxObj[p] + p.length, bold);
        });
        cell.setRichTextValue(val.build());
      }
    });
  });
}

SpreadsheetApp.newTextStyle

This function is modified to search for words in column A that are selected from words in column B

function boldMywords() {
  //const red = SpreadsheetApp.newTextStyle().setForegroundColor('red').build();
  //const org = SpreadsheetApp.newTextStyle().setForegroundColor('orange').build();
  //const blu = SpreadsheetApp.newTextStyle().setForegroundColor('blue').build();
  const bold = SpreadsheetApp.newTextStyle().setBold(true).build();
  //const cA = [red,org,blu];//colors array
  //const wordA = ['coffee','phone'];
  const ss = SpreadsheetApp.getActive();
  const sh = ss.getSheetByName('Sheet0');
  const rg = sh.getRange(1,1,sh.getLastRow());
  const wrg = sh.getRange(1,2,getColumnHeight(2,sh,ss));
  const wordA = wrg.getDisplayValues().flat();
  let vs = rg.getDisplayValues();
  
  vs.forEach((r, i) => {
    r.forEach((c, j) => {
      let idxObj = { pA: [] };
      wordA.forEach(w => {
        let idx = c.indexOf(w);
        if (~idx) {
          idxObj[w] = idx;
          idxObj.pA.push(w);
        }
      });
      if (idxObj.pA.length > 0) {
        let cell = sh.getRange(i + 1, j + 1);
        let val = SpreadsheetApp.newRichTextValue().setText(c);
        idxObj.pA.forEach((p,k) => {
          //val.setTextStyle(idxObj[p], idxObj[p] + p.length, cA[k % cA.length]);
          val.setTextStyle(idxObj[p], idxObj[p] + p.length, bold);
        });
        cell.setRichTextValue(val.build());
      }
    });
  });
}

function getColumnHeight(col, sh, ss) {
  var ss = ss || SpreadsheetApp.getActive();
  var sh = sh || ss.getActiveSheet();
  var col = col || sh.getActiveCell().getColumn();
  var rcA = [];
  if (sh.getLastRow()){ rcA = sh.getRange(1, col, sh.getLastRow(), 1).getValues().flat().reverse(); }
  let s = 0;
  for (let i = 0; i < rcA.length; i++) {
    if (rcA[i].toString().length == 0) {
      s++;
    } else {
      break;
    }
  }
  return rcA.length - s;
  //const h = Utilities.formatString('col: %s len: %s', col, rcA.length - s);
  //Logger.log(h);
  //SpreadsheetApp.getUi().showModelessDialog(HtmlService.createHtmlOutput(h).setWidth(150).setHeight(100), 'Col Length')
}

This one will provide case free matching:

function boldMywords() {
  //const red = SpreadsheetApp.newTextStyle().setForegroundColor('red').build();
  //const org = SpreadsheetApp.newTextStyle().setForegroundColor('orange').build();
  //const blu = SpreadsheetApp.newTextStyle().setForegroundColor('blue').build();
  const bold = SpreadsheetApp.newTextStyle().setBold(true).build();
  //const cA = [red,org,blu];//colors array
  //const wordA = ['coffee','phone'];
  const ss = SpreadsheetApp.getActive();
  const sh = ss.getSheetByName('Sheet0');
  const rg = sh.getRange(1,1,sh.getLastRow());
  const wrg = sh.getRange(1,2,getColumnHeight(2,sh,ss));
  const wordA = wrg.getDisplayValues().flat();
  let vs = rg.getDisplayValues();
  
  vs.forEach((r, i) => {
    r.forEach((c, j) => {
      let idxObj = { pA: [] };
      wordA.forEach(w => {
        let idx = c.toLowerCase().indexOf(w.toLowerCase());
        if (~idx) {
          idxObj[w] = idx;
          idxObj.pA.push(w);
        }
      });
      if (idxObj.pA.length > 0) {
        let cell = sh.getRange(i + 1, j + 1);
        let val = SpreadsheetApp.newRichTextValue().setText(c);
        idxObj.pA.forEach((p,k) => {
          //val.setTextStyle(idxObj[p], idxObj[p] + p.length, cA[k % cA.length]);
          val.setTextStyle(idxObj[p], idxObj[p] + p.length, bold);
        });
        cell.setRichTextValue(val.build());
      }
    });
  });
}

This last one get's the words to search for from a prompt:

function boldMyWords() {
  const red = SpreadsheetApp.newTextStyle().setForegroundColor('red').setBold(true).build();
  const bold = SpreadsheetApp.newTextStyle().setBold(true).build();
  const ss = SpreadsheetApp.getActive();
  const sh = ss.getActiveSheet();
  const rg = sh.getDataRange();
  let vs = rg.getDisplayValues();
  let r = SpreadsheetApp.getUi().prompt('Enter words to search for separated by commas', SpreadsheetApp.getUi().ButtonSet.OK_CANCEL);
  if (r.getSelectedButton() == SpreadsheetApp.getUi().Button.OK) {
    let wordA = r.getResponseText().split(',');
    vs.forEach((r, i) => {
      r.forEach((c, j) => {
        let idxObj = { pA: [] };
        wordA.forEach(w => {
          let idx = c.toLowerCase().indexOf(w.toLowerCase());
          if (~idx) {
            idxObj[w] = idx;
            idxObj.pA.push(w);
          }
        });
        if (idxObj.pA.length > 0) {
          let cell = sh.getRange(i + 1, j + 1);
          let val = SpreadsheetApp.newRichTextValue().setText(c);
          idxObj.pA.forEach((p, k) => {
            val.setTextStyle(idxObj[p], idxObj[p] + p.length, red);
            val.setTextStyle(idxObj[p], idxObj[p] + p.length, bold);
          });
          cell.setRichTextValue(val.build());
        }
      });
    });
  }
}